I ask that, since it appears there was no hacking involved here, so I assume this is an account she had access to from when they were together.
Imagine it was the keys to her personal apartment she had in her name during marriage. It would not be ok for her husband to enter there after they were divorced.
Because if it is actually a crime, me and my partner have been committing that crime on some of our accounts.
Personally, I don't find it to be criminal no. Like giving keys to my appartment to a friend when I'm away on vacation to check on my dog. Obviously, if they come in my house and steal things that's different, but if they were to come in for some seemingly reasonable reason and I didn't want them doing that anymore I should ask for the key back or change my lock.
A divorce can be a pretty nasty thing for all parties involved. Had you shared your bank account with your significant other in an informal way, such as giving them your user/password for it, but not actually made them an official account holder, and during the divorce if the partner signs in for something seemingly reasonable, like making sure there's enough money to pay the bills, I also don't find that criminal. You probably need to have a talk about this, and might want to change your password if you don't want this happening anymore. Again, would be very different if money was transferred out and stolen.
Of course, I'm old enough to remember when cereal box offers would say "Tape four quarters to a piece of cardboard and mail that in with your request as payment for this offer."