your profile does not indicate whether you work at a gym or by yourself. I assume the latter because gyms easily have trainers of both genders available.
I would assume that there are only some parts of the assessment that would require adjustments. After all, the client asked you to work with her. If you cannot ask another trainer, yo could ask the client to have some assessments done by a friend or relative, such as body measurements. If you are used to do caliper assessments, those may have to be eliminated because this cannot be outsourced. Most other assessments can be done visually without touching a client.