Be brave! Just do it!
It is possible to switch between the mobi and epub firmware without problems and you don't need anything more than the two firmware files.
(To comfort your friend a little bit: ask her if she would upgrade her Gen3 weekly or monthly if Bookeen would offer a new mobi firmware every week or month. I think she would, and the procedure is technically the same if you upgrade mobi on mobi or epub on mobi and back. And because I'm curious I switched from mobi to epub and then back to mobi again and didn't experience any problems.)