Um... Geez haven't you guys ever taken a health class or human sexuality class or anything?
There is not really such a thing as a "dormant" yeast infection for a woman... the symptoms ARE the infection... if there are no symptoms then it has gone away. It's not like herpes where you have it but only get outbreaks at certain times. It's not a lasting infection. She may be prone to them, which is why she has RECURRING yeast infections... but it's not one single infection that dies down and then "flares up" again.
And you don't "contract" a yeast infection. It is not a sexually transmitted disease... and it is not viral. It is caused by a bacteria called "candida albicans" which is normal to have some off at all times but when they multiply rapidly it causes candidiasis (yeast infection). They can be caused by a change in pH, increased heat and moisture, allergies, elevated sugar levels, hormonal changes, weakened immune system, reaction to antibiotics, etc.
It is possible for candida to be transmitted between people by direct contact, so unprotected sex isn't necessarily a good idea (and the spermicides on some condoms can also bring one on if the girl is particuarly sensitive to them). It is possible for a woman with a yeast infection to "infect" her boyfriend, treat her infection so it goes away and then get reinfected by him again. (Men don't usually have recognizable symptoms, so they wouldn't know they had it.)
Sex would probably be painful for her during a yeast infection anyway, so I'd say wait out the week it takes to treat it without any sex or going down on her (which is just gross). If you have already had unprotected sex with her maybe ask your doctor about it, but I wouldn't worry about "how did she get this?" Although it's possible to be passed sexually, it is NOT an STD.
she noticed she had a burning 'down there' and she remembers this "burning" when she used to have it years it ago, so she suspected maybe she got it again. Understand now?