Your client's sudden recovery isn't necessarily surprising. My experience has been that children sometimes respond to treatment much more quickly than adults typically do, in part because their problems are less ingrained and in part because pre-adolescents often are less skeptical about my explanations that adolescents and adults are. It may be that, once she understood that it was not necessary for her to avoid cockroaches, she was able to handle the exposure on her own.

Why didn't she want to meet with her therapist for an appointment that she saw as unnecessary. It could be simply because she's a kid who didn't see any reason to meet with a therapist if the problem is gone.

As far as whether there is a need to try to do something to prevent OCD, from the information that's been presented I'm not sure we're dealing with OCD. It sounds as though the client was taught explicitly by her mother to avoid anything "dirty" for fear that she would contract another infection and it does not sound as though she has developed any obsessions or compulsions. It certainly could be useful for the therapist (or her parents) to help her realize that she no longer needs to avoid items that seem dirty and to help her understand the value of facing one's fears rather than avoiding. However, I'm not sure she's particularly at risk for developing OCD.