The best we can do is offer you the information that is here about why not to keep snakes together. We can't go tell her not to keep her 3 snakes in one tank. The information is all over this site as to why Not to keep snakes together, and you can show it to her, and she can continue keeping them together and risking all those things happening, or she can seperate them, risking that they will "miss each other" even though no one here that I know of has ever had any Bad thing happen from seperating snakes housed together.
It's a question that is asked all the time, so there's TONS of threads, and they all say the same thing.
"Here's 50 reasons you should not keep snakes together"
"Well, I want to keep my snakes together anyway."
Repeat that about 5 million times. On occasion someone does seperate their snakes, and report better feeding, more relaxed animals, etc.
Send your friend to those threads, so she can read them. Be prepared for her to ignore it all, just like tons of others. There's still folks that swear by unregulated heat rocks and pine bedding for snakes too, and they are just as wrong.
Thanks for trying to suggest a better way to keep them to your friend.