The girl is clingy? She doesn't have friends? Those are two big red flags for me.
I hate to say this and this might sound pretty chode, but your friend and his girlfriend aren't each others' types. Although I generally refrain from discussing relationships on this forum, I believe that you can reliably measure how well two people match for a relationship on how similar they are in levels of maturity. It's not so much of the age gap that's causing this issue; your friend is mature enough to see the value of independence while his girlfriend hasn't grown out of the daddy issues she most likely has.
I think your friend should find a girl who's more focused on her independent passion(s) in her life.
I think your friend's current girlfriend should find a guy just as codependent as she is. Either that or she should discover what her passions are and enrich her life in the areas of health, wealth, and relationships according to her core values so that she can mature a bit. Yes, a part of that may involve you helping her make friends... if you want to leave her better than you found her, Bonita.
Girls whom I've thought have gotten a little too clingy usually get the hint when I'm contacting them less and less and when I'm giving a more distant and "friend" vibe... but I guess that won't really work for 4-5 month relationships.