1) Unless 95% of the traffic stays local to the same subnet, I would
_not_ use a PC as a router.
Having a slow WAN link bottleneck on the other side could also qualify?
2) If you don't need segmentation, then "supernet." _All_ systems can
keep the same IP addresses, just their subnet masks need to change.
I think he needs to be told of the implications -- masks are not to be
slipped off their proper left bounds. You can't shorten them beyond the
class' network part and you can't (shouldn't) step on other people's
networks. As IIRC we don't know if he has private addressing, this
should be done under a network engineer surveillance.