The 15 and 64 are the same frame size. It's possible a round butt might fit her hand a little different than a square butt.

It looks like you're in search of an answer for a problem that doesn't exist.

What is she going to do if her 64 breaks. While the repair time will be much shorter it will still likely be out of service for a couple weeks. Will she go without? If not, and she'll have another gun to use then just have her use that gun should the Colt break. However, use that backup till you can get her a suitable replacement.

It'll take you about the same time to buy a replacement S&W for her Colt as it will to have the 64 fixed. So, today, you're left with the exact same problem you would have if she had a 64.

I don't see that you've solved anything by replacing what appears to be a great choice *for her*.