Most Helpful Girl
That seems super sketchy. I think she was trying to cheat unfortunately or a least flirt/sext your friend on snapchat. I would either confront her about it, or if your friend is up to it, have him flirt with her on snap chat and see her reaction. Then go from there.
Most Helpful Guy
She was flirting and was making an obvious point of telling him she was alone, which I would see as an invitation for him to make her not alone! Ask her about it and see what she says. How would she react to you doing it to her friend?