It is not true that people cannot - or do not - interact successfully with people that are less intelligent than they are. Many children get along well with their younger siblings. Many adults love being kindergarten teachers... Or feel highly engaged working in the dementia wing of the rest home. Many people of all intelligence levels love having very dumb pets. These are not people (or beings) that you relate to because of their 'relatability' in the sense that they are like you, but because they are meaningful to you. And interacting with people build social skills appropriate to those people -- which may not be very generalizable when you are practicing interacting with kindergarten students, but is certainly a useful skill when you are interacting with average people.

I personally would think that the problem under discussion is not related to intelligence, but in trying to help an introvert identify the most fulfilling interpersonal bonds without making them more social in a general sense. However, I don't know the kid in question, so I can't say.