I really hope that your brother keeps his PSP on 6.60 and just uses Fast Recovery if he powers his PSP off completely (the PSP remains in (L)CFW mode when it's just put into sleep [i.e. pressing the power switch up quickly and not holding it]).
It's not worth the bugs and the possible brick risk of downgrading a 9g by faking it as a 4g to 6.20 just to install the perma patch (which also has a slight brick risk).
I think he'd much rather have a 100% working PSP than a 10% or 20%*
risk of bricking it all because he was too lazy to wait the 6 seconds (Fast Recovery is actually that fast, hence the name "Fast Recovery"
) for PRO to load if he completely powered off his PSP (which is never really needed in the first place).
That's just my opinion though, in the end it is his device and he can install anything he wishes.(*The numbers might be slightly exaggerated but the message is still the same)