My mom also felt the same way, but I think it has a lot more to do with the name then the actual idea of it. Once I showed her some pics and she met my photographer she became more excited about it. The photographer we are bringing with us also calls it Anti-Bridal which helped her get over it as well!
I agree with the others also though, if she still has these feelings wear your short dress for TTD or buy a cheaper dress, there are lots of options!