personally.. i don't think this is something to worry about.
I don't think his gf is interested in the other guy or anything.
If she just mentioned his looks, well she might think that he looks great, but it just seems like a comment more than anything else.
It's not like you see a pretty girl on the street and you want to be w/ her and dump your girlfriend!
I think if she's open enough to talk about your friend being hot; then she's assuming that you're open-minded enough to just listen to her w/o being too sensitive.
I wouldn't confront her or anything and tell her how uncomfortable that makes you feel-- cuz she'll be so careful w/ her words being with you-- and that'll just ruin the time you guys have together. Like what others said, just go along with her.