Well of course there is prescription medication like Viagra, but I'm not sure if those types of medications cause muscle damage. They are FDA approved, and if there were many cases of muscle damage, I think we would be seeing some class action law suits.
What your buddy could try is a natural remedy called Epimedium (another name for it is Horny Goat Weed). Epimedium is an extract from a plant that grows in China and parts of Europe. I have a buddy who has tried this and he said it works really well - almost too well. Tell your buddy to give it a try. It may just do the trick.