If you are still on good terms, then the simple solution is to add her to your Household Account.
Before each redemption, you zero out your own personal Avios balance by sending it to IB+. Then the Avios will be taken from her account.
This avoids your ex:
- Paying the transfer fee
- Paying the taxes / surcharges on a redemption out of her account
(both of which you are presumably reimbursing, but still...)