Agree with Bill. The rim doesn't have any bearing of the hub (except for spoke count). So if the hub/axle is in good shape there's no reason why a beefier rim couldn't be built up on it. But before I'd suggest that I'd want to know more. If the hub is a freewheel one then the asymmetrical bearing position and resulting axle bending potential (more rider weight = more possibility) is one long term concern I'd think about. Parallel to this is that most freewheel hubs, these days, are made of fairly low grade materials so bearing wear over the miles is another concern. Next up is the spoke count. Lots of bikes come with 32 hole wheels, not 36. More spokes mean each spoke does less work and when (not if) the next spoke breaks the ability to retrue the wheel is greater with more spokes.

The cost of a new rim, spokes and labor isn't a small amount and it would be a shame if the "new" wheel needed replacement sooner then later due to hub issues. At the weight of the rider he will continue to have spoke and rim concerns, it's not like there is a black and white line of everything is fine and 1 pound more weight or 4 spokes less and all goes to pot. The goal is to extend the period between the problems best possible.

A cassette hub, 36 quality spokes and a stout rim built by a competent guy who can be revisited as need is the best path, IMO. Andy.