Yes, you can return to 6-MP once you stop it. BUT, remember, it will take somewhere between 2 and 6 months to start working again.
I will point out, that although some people stress over 6-MP when they first read of the side effects, they forget to compare that to Prednisone, Remicade and the other drugs we take.
By comparison, 6-MP is a very safe drug with an extremely low complication rate. If your daughters GI is monitoring her properly, they will be ensuring that her white cell count is stable and that metabolite levels are within a safe range.
I will also point out that your daughter is on a rather safe dose of 6-MP. Many of us on this forum have been on doses as high as 200mg PER day.
The decision is your daughters but I would very strongly suggest that you both discuss changes to her medications with your GI as there is a great deal of IF this THEN this when it comes to UC.
I am so glad your daughter is getting better.