I'm in a PhD program in the life sciences and although I haven't graduated myself (still in my first year), based on almost all of the advice I've read, this is good advice. The way to prove to your advisor that you deserve to be able to do quick secondary experiments with people outside of your field is to submit a paper of your own, and then you can do those experiments in the interim while you're waiting for the reviews. That said, I don't think I'm all that Slytherin (although I admit that this is what a Slytherin would say).