Off label use is fine. I'd be more worried about the fact that a patient is bring it up out of the blue (ie. "give me drugs plz"), and the evidence for life extension is shaky

>There is some evidence metformin may be helpful in extending lifespan, even in otherwise healthy people. It has received substantial interest as an agent that delays aging, possibly through similar mechanisms as its treatment of diabetes (insulin and carbohydrate regulation). This is controversial, and an area of active research.[61][62]