I heard that from a friend that played KH2FM+ on critical. So let's say Sora has 120 HP. In critical mode, Sora would only have 60 HP. I believe that is the same with Goofy and Donald as well about only obtaining half the HP but I'm not sure though. =\
Edit: Well...I do know on Critical mode, enemies do 2x the damage on you compared to Standard Mode. That is the same with proud. And on page 34 of the KH2FM+ Ultimania it says on the 24th change in a box on the top right about HP and MP. I think they both might be decreased by half but my Japanese isn't good enough to read the whole sentence. =\ But I think that's what it says. @_@