"Be transparent, but be brief"
I don't remember where I found that quote but I have never forgotten it. Be completely open and explain (briefly) what happened. She'll appreciate it.
Also, you should have some sort of documentation regarding the license. For instance, if you are an Emsisoft Reseller then check your dashboard. Even if you are not a reseller, there should be a digital trail for the software.
Now, let's assume things go south and you get pushback from her. At this point you should ask yourself if you want to continue working with her. In the past I have dropped customers (nicely) that were more hassle and headache than they were worth. It's something to consider.