A lot of cons don't allow this due to the convention centres they are in don't consider these shoes as the bottom is soft and offers no protection. I don't know about fanime specifically but people from other cons have been getting the no go on it. If you do wear them though, aside from being soft, they also don't have any support so they are going to hurt a lot after a while. It's basically walking barefoot. Since she is going with body paint I know he options are harder and maybe a tad off but her comfort should be most important. With flip flops, you would need good primer and then just touch ups here and there. But it isn't even that important to touch up since most people probably won't be looking at he feet anyways or long enough no notice parts are lighter. There's also this other show called iguaneye which are considered shoes which you can look into. They are also reusable which is better then the sticker shoes. Another's option is to make invisible shoes. These have straps also but they are see though. You can also paint the mesh the skin color. These also don't have support but atleast they have a hard bottom to protect your girlfriend.
Forgot the link: http://andsewingishalfthebattle.com/invisible-shoes/
Forgot the link: http://andsewingishalfthebattle.com/invisible-shoes/