If their current site uses that structure, that’s understandable.. if it’s for SEO purposes, then I don’t think that this will affect their ranking much.
If their site is not very big, what you can do is set a redirection on the old links to the new structure, that way nothing will break, but that will only work if their site isn’t too large. Creating hundreds of redirects wouldn’t be good.
Hopefully someone else will come along and provide feedback. I’m not entirely sure if getting that desired permalink structure is possible.