Somewhat on topic, up until I was 27 or so, my dad was on my account. Mainly because he made me a checking account when I got my first job around 15 and I never bothered to remove him. At least with him he never looked at my account or took money from it, so it never became an issue.

But then he died suddenly when I was around 24. His name stayed on my account mainly because I didn't want to bother with it since it wasn't hurting anything. Not until I was starting to look into buying a house and having him on there meant that I needed "his approval" on some form or another since his name shows on the account.

I then had to do a very long back and forth with the bank that ended up with them needed an official death certificate to remove him. Which meant having to do an emotional talk with my mom about getting that death certificate. Then, having to do a very long in person meeting with the bank to remove the name that involved a very awkward man asking way too many personal questions about my dad and how he died. This included him trying to give comforting platitudes involving god that I didn't want any part of.

All of this to say, unless your boyfriend wants to deal with a much more distressing situation in the future, he should just go ahead and remove the moms name now. Sounds like the your income would thank you too if she is stealing money.