My friend is the same way and no matter how many times I tell her or how many times I sent this forum to her, she still doesn't change anything. Maybe you can take videos and pictures of him while he's at your house and show your friend how happy he is living in this condition, so she can either realize and change or give him to you permanently!
I have also commented on peoples pictures saying that "those treats are bad," "they shouldn't have salt or mineral licks," "vitamin c drops shouldn't be given and are pointless," and "that C&C cages are great for guinea pigs, maybe you can try one."
^Some one posted a picture of their new cage (which the same as my old pet store cage, I use it as a transporter, and only use it as an actual cage for when we go on vacation) and they said how they finally had enough money to buy their dream cage. I thought to myself, you probably could've built a C&C cage for less money. I nicely told her that C&C cages are bigger and better and that two guinea pigs need at least 76x127 cm of space. She rudely said I take care of guinea pigs how I think they should be taken care of. Like I understand but you aren't taking care of them correctly and I want you to realize that. Another person posted a quick video of their morning routine. They had a proper sized cage but crappy pellets. Me and someone else told her that guinea pigs need 1/8 plain Timothy pellets and that colored bits are bad for guinea pigs and result in picky eating. She said that we shouldn't be telling her how HER guinea pig should live and that her vet recommend those pellets. Pretty sure it wasn't an exotic vet, because they know they get plain pellets.
I just wish people would realize guinea pigs need so much specific care and that people will actually give their guinea pigs that care.