Go to another account she hasn’t messed up on her phone, and make her watch as you use the password manager to get in. Then, you can tell her for sure that the tech is working, and you’ve done your part, but you cannot fix her behavior. If she wants to keep resetting her passwords all the time, that’s on her, otherwise, she’ll have to put a small amount of time and effort into adapting to using the password manager.

If she isn’t going to follow your suggestions and advice, why is she asking you for help? If she sincerely wants help, she needs to make an effort on her side to follow through.

This is a problem with psychology and boundaries, not a tech issue.