Go for it, have some fun. If it doesn't feel right when it's happening you can always just walk away! Sex doesn't have to have an emotional connection. Get to know her and she might surprise you.
Establish Boundaries, make sure you are all happy, hes seems to talk alot for his gf, she might not be as willing as he is
Don't get yourself emotionally attached to the whole thing. Be available to them to satiate your curiosity but have your regular sex life outside of them. If it gets complicated you walk off. You're already talking about the girl having his dick while you are "dickless." Wrong approach