If he didn't provide full spectrum lighting then he couldv'e given her supplements which is just as good. Largre day geckos species usually take 14-16 months to become totally mature and may still grow a few more inches after that. As long as you are giving her the right lighting and supplements she should grow more.

I don't see how adding wet towels in a bowl would be stressfull. If you constantly have to change out the towels more than once per day then you may want to do something else to increase the humidity. If you have the cash the best way to achieve success is by following this guide on how to build a reliable misting system;
Mister Guide