It depends what car he's getting. If he's getting one of the old 55EL's then yes there isn't really much advantage over the 5 speed box, just a quieter motorway gear and slightly better acceleration. Yes it will cost more to service, just. Insurance is unchanged.
If he's getting the mk2 Sporting (which it sounds like as he says 1.2) then it's insurance is group 7, teh box has no bearing, and here the 6-speed box improves performance noticably by keeping the engine in the sweet zone.. It's a good gearbox and you shouldn't be put off by worrying what may go wrong with it. Any gearbox can develop issues with age, this one is no more or less susceptible.
Spirito di Punto | M-reg 55SX, black, 5dr. Clear repeaters. Remote locking fitted, no light confirmation yet though - need relay fitted