The 208 that you refer to is the "high-leg" voltage to ground on a 240/120 volt 3 phase delta connected transformer secondary with on of the windings tapped at midpoint to provide 120 volts. The voltage of one of the phases, usually the "B" phase is about 208 and obviously should be avoided if connecting 120 volt equipment. There is also a wye connected 208/120 3 phase system where all 3 phases are at 120 volts to ground.
I would not use 208 heaters at 240 volt. But, most of the ones I've seen are dual rated. If they are not dual rated, have him get 240 volt heaters. BTW, you probably do not have two-phase voltage. It is very uncommon these days. You must mean 240 volt single phase with a neutral. This is the standard residential system.
Doing a little basic Ohm's Law here (ignoring the fact that this is AC not DC) you'll find that a 5000 watt (I just used this value for comparison) 208v unit draws 24 amps and has ~8.67 Ohms resistance. The same unit at 240v would draw ~28 amps and put out 6720 watts (R basically stays the same). Their life span will be real short.