You've already got two things going for you.
1. The current lead tech is available to you, so you're not completely in the dark.
2. Because you will have to take care of the documentation yourself, there is less probability of gaping holes with lack of information, because you will be asking that lead tech questions like crazy to ensure you have a viable reference guide for when she is gone.
What make/model of Citrix are you going to be working with? What is your familiarity level with Citrix environments in general, and the type of network that your company has? These questions will help many people provide answers at a level that you need, rather than being too basic or too advanced.
Best of luck to you!