Straight Edge has nothing to do with prescription drugs, as long as you're not abusing them or using them recreationally, of course. It's about not falling into a lifestyle of addiction or chemical dependency (some straight edge individuals don't have recreational sex either).
He can use medicine with or without a prescription.
Pretty much this. Straight Edge isn't a black-and-white lifestyle, but rather, a very flexible way of life. Not to draw attention away from the topic to myself, but I'm also Straight Edge. I've had to take pain killers and antibiotics for a particular surgery I had, and during that surgery I was, of course, put under. I pretty much just avoid illegal drugs, alcohol, and nicotine/tobacco products (and sometimes wear douchey pro straight-edge shirts).
To answer your question OP, it really just depends on how strict his regimen is.
