Sounds like she should sign up for an account with a registrar she'd like to use, then initiate a transfer request with that new registrar for the domain name, that would get it out of your hair and into hers.
There may be the possibility of allowing some control by creating a new panel user for her then giving her just the acess that she'd require, see:
for details. Not sure you'll have luck defining permission down to the level you'd like, especially in the case of the billing question. If you can't do via those means, frist^ paragraph is your only hope, and probably the best option to begin with for all practical purposes.