I'd start with gastroenterologists. I've started having some GI issues recently so I've been seeing one, but there was a 4+ week wait time to see anyone in my area.

Seconding a gastroenterologist. In Australia you have to be referred to by a general physician, and the GE will perform the colonoscopy procedure and also consult. I'm in my 30s and had mine done just the other week. The prep sucks but it's for a good cause, and the anaesthesia is divine.

Good on your friend ;) It's important to look after your brain's meat suit, particularly that area considering the growing body of research between the gut/brain