So here is the thing, most doctors will tell you that you will be fine, as transmission happens when several conditions are met namely, high number of infections agents, lowered immune system of the individual and lowered local immune system, that being said, candida is a lifelong passenger with humans, so I would give much thought. Also B vitamins with combination of high dose c vitamins or lemon juice would be more effective for oral candida as it will tilt the pH balance towards base which is inhospitable for yeast.
Is her tongue white? Thrush doesn't usually manifest as a sore on the outside of the lips, as candida requires a moist environment to survive. Just wondering if this has been diagnosed and if the other symptoms of thrush are there because if it's just this sore again are you sure it's yeast? Not saying it isn't, but you do want to be sure because those anti-yeast treatments can cause future yeast infections by killing off beneficial bacteria that prevent overgrowth of yeast in the first place -- candida is always there, it's just usually under control by the good guys. Many antibiotic and antifungal and steroidal meds kill off the good guys, which is one of the main reasons we get yeast infections. At any rate, it's highly unlikely for a guy to get a yeast infection, although it does happen. The reason is anatomy -- the vagina is very open and accessible, the opening in a man much less so. Again, yeast needs warm moist conditions to survive and needs a die-off of things that protect you for an overgrowth. Men do get it, but not often. Oral thrush is a more likely thing for a guy to get. If you're a typical guy who has had some amount of sex you've undoubtedly had contact with women who had yeast infections and I'm betting you didn't get one, so I wouldn't really put this one on your worry list.
Personally, I'd wait until it's gone, if for no other reason than it might not be very comfortable for her to do it if her mouth is sore. The risk is probably low, but go with her comfort level.