American medical wages are above the world average, but medicine is one of the highest paid professions throughout the developed world, including in Canada (and yes, moreso than pharma PhD researchers on average, especially if one pursues a more lucrative specialty). Probably this should not be the determining factor.

The outside view would be that probably American medical wages will not plummet in the next 6-8 years, although that's a fairly irrelevant question (a career extends for decades).

or goes to a medical school in the Caribbean (where getting into them is apparently easier than mainland medical schools, but to complete the training takes six years)

These schools tend not to do as well in placements for residencies.

I'm a doctor working in the UK, a few points.

1) As Carl notes, Medical wages in the US are particularly extravagant, but they are still pretty high in other places in the anglophone world, and high generally. Carl has done more research on this than me, but moving to practice in the US has significant transaction costs, which may make moving not-that-great on expectation. In summary, the short- and medium- term changes in medical reimbursement in the US shouldn't be a dominant consideration.

2) It is not clear to what degree medical wages are inflated. Ex ante, you'd be surprised if the optimal model of healthcare was designed around an elite corpus of highly skilled people people (doctors) in charge of almost all aspects of patient care, and involved in almost every interaction a patient has with the system. Corrections to that (in terms of increasing automation, division of labour with easier tasks handed off to lower-skilled staff) seem to be brewing in most healthcare systems, and may have a downward effect on wages. There's also the effect of immigration reform exerting further downward pressure on wages if the medical guild's protectionism can be broken. These may also give opportunities for leveraging these things, leading to possible increased variance in returns on medical careers (the 'job for life' model where everyone becomes a consultant/specialist of approximately similar rank may go, with 'superstars' presiding over more junior staff).

That being said, it isn't clear whether doctors are a poor value proposition at the moment: some very speculative research I've done on the marginal health impact of additional doctors puts their 'cost per QALY' in line with marginal health technology expenditure, at least in the UK. Also, being a doctor is demanding across a variety of axes (intelligence, domain knowledge, social interaction), so they are more robust to disruption than most other jobs. So I doubt any dramatic change in salaries for doctors in the developed world anytime soon, but I'd predict they will go down rather than up.

3) Medical schools effectively select for intelligence, conscientiousness, integrity/appropriate behavior, social skills, and dedication to the profession. Due to competitiveness, medical schools can select far from the right tail. I'd guess there are better good opportunities in other EtG paths than medicine for any given level of ability, especially given the high upfront costs.

The cases where I think it would most likely be an optimal fit are for people who are 2ishSD above the mean in IQ, very conscientious, and good (but not exceptional) 'soft skills': you are smart and hardworking enough to have a good shot at medical school, but you aren't smart enough where you have a good chance of 'winning big' at a g-loaded tournament game (start-ups, finance, STEM), and you aren't socially good enough to win big at socially loaded tournament-games like business/entrepreneurship either.

4) The 'soft factors' are important for getting through medical school and staying in the job. You spend your time learning to memorize and apply large amounts of factual data. On the job, you need to be able to interact well with patients and colleagues from all backgrounds, you need to cope with episodes of high physical and emotional stress (examples from my first month of being a doctor: telling a patient they were going to die, telling a relative we were stopping active treatment of her husband for 60 years, being the first responder to a patient who had thrown up a liter of blood and was still doing so, CPR with relatives in the room screaming, trying to talk an extremely agitated person in alcohol withdrawal to let you give them drugs before they go into a life-threatening seizure, lots of seeing and examining dead bodies, body fluids, and body parts); you also need some threshold level of manual dexterity to perform basic procedures like taking blood etc. There are lots of upsides to being a doctor - I really enjoy it - but I think there are lots of people who would struggle despite being smart, hard working, and genuinely invested in their patient's wellbeing.

5) There are fair exit opportunities for medicine (Pharma, consulting, finance), so you aren't 'locked in' to a medical career.

There are a couple of important questions you didn't raise.

Does he like working with people? Does he perform well under social pressure? Is he good at explaining things in simple terms? Would he like doing so every day? Would he like working in a team and maybe leading it? Would the strict social hierarchy of hospitals bother him? How does he handle sleep deprivation? Can he cope with constant uncertainty about his decisions? Can he handle killing/injuring people by accident?

he is very passionate biology and the study of life

Med school isn't generally about that. Would it be agony for him to memorize loads of facts without questioning/understanding them too much, then forget them because he doesn't need them for anything? Also, much of the stuff you have to memorize after the first 1-2 years has nothing to do with human biology. There are some challenging moments with complicated patients, but the work is mostly quite simple and algorithmic.

to biology, because he no longer believes he wants to become a programmer, and because he is very passionate biology and the study of life, and he would rather go into grad school for biology, or maybe medical school.

Doctors don't engage into the study of life. That's not their role. Does he want to be an academic?

There are plenty of biology research projects where programming skills are very useful.

Figuring out expected future wages is a fool's errand. He should do what he likes and good at. If he is passionate about biology, and not medicine, than he should stay away from medicine. A biologist in a pharmaceutical company is paid quite well and lives a lot less stressful life than a medical doctor.