Yeast infections can be common in girls. Most women get them thoughout their lives, and men can get them too. It CAN be passed with intercourse, but it's also caused by a natural buildup of yeast bacteria. Even her diet can cause them.
I wouldn't recommend having sex with her during the infection. First off, you can pass the infection between yourselves while trying to get rid of it. Second, it's kinda gross. Depending upon how bad the infection is, it look cottage cheesy. Plus the women's medication is goopy, and so she's sure to be self conscious.
I've had one, from my wife, and we spent several weeks passing it between each other until we abstained from sex long enough to be certain we were both clear of it.
Give her at LEAST a week after she says she's cured before having sex.