all rats have myco, they're born with it. if they have not had a flare in a while then your rats should be just fine. how long is a while though? and when they were sick how servere was the flare up? also, how healthy are they now? are they completely normal or was there lasting damage? if there was lasting damage then it can sometimes be confused with a small flare as well and the rats may not be as healthy as the owner thinks. if they are completely fine now and have been for at least a month then i would say your rats would not be at risk if you were rat sitting for her. i'd be more concerned about mites but i'm sure she would know if her babies are itchy. it is always a risk to bring any new animal or object into the home but you can't live in fear and should weigh the chances accordingly. if you trust this lady and what she tells you seems acceptable then go ahead, if you have any concerns about it then discuss them with her. i'm sure she will apprecaite your up-front-ness with her and will respect your concern for your own rats (after all she's a rat lover herself).