Is there a way to give sub account users

The ability to create folders or at least to drag forms into folders?

Yes that is possible and quite easy.

I would suggest checking the following guides on this:

How to Share Forms with a Sub-Account User

How to Add or Group your Forms into Folders

Do however let us know if you happen to have any issues or any questions along the way and we would be happy to assist with the same.

Ok. I guess my sub account user was probably looking under her shared forms rather than her own forms.

If she has any issues, do let us know - or ask her to let us know and we would be happy to assist with the same.