I think she actually MUST have a different email address than you do. Wikitree uses the email address to distinguish between different Wikitree members.
I think she has to join using a different email address, go through all the joining process, confirm the email address, etc. Then, when she is a member of Wikitree, you'll have to add her to the "trusted list" of her own profile. Then there will be two versions of her profile (the one created when she joined, and the one you created previously). Those will have to be merged so she becomes one person. If she needs help, you may need to ask her to add you to her trusted list as well. That way you can go into her profile and help her.