If your partner is prone to yeast infections you need to see a doctor about it together. I've heard of cases where the woman had the yeast infection, gave it to the man who had only mild symptoms, while she was treated. And then once the treatment was over, he gave it to her over again.
Aside from treatment, the best way to prevent this is to use a condom while she's infected (and while he's infected as well!).
Otherwise you'll have and itchy vagina and an itchy penis together and that's not fun at all.