swedish

I would think she should already realize this since he has a girlfriend and the minute they discussed having his buddy join them, that should be a big red flag that this is about sex. If you fear she has fallen for him emotionally, maybe talk to her again about slowing it down because you don't want to see her get hurt. Is he saying things to her to give her the impression it's more than sex? If not, she probably knows what she's in for and choosing to ignore it. If he is, tell her you are worried he's saying what she wants to hear because he's getting his cake and eating it too...maybe advise her to back off and see if he still says these things when easy sex isn't on the table.