If they are thinking 50" plasma then their budget is well over $2,000 unless they want absolute bottom of the barrel stuff. So, I would think the budget is a little bit more. In my experience, plasma and LCD flat panel displays tend not to look as good overall as rear projection models that are DLP, LCD, or LCoS... So, unless they want to hang it on the wall or have no space for the larger display... or they want to free up some floor space, I would go with a RPTV.
Now, DLP, LCD, or LCoS? DLP is excellent, but I would take a long hard look at the Sony SXRD (LCoS) rear projection models as well. Some may disagree, but they have IMO one of the strongest images you will see of any rear projection (or front projection) technology. Especially if they have been happy with Sony over the years, they may find some comfort in sticking with a brand they trust. My brother just got the new 60" model (per my recommendation) and he called after watching for a couple of weeks to say how psyched he is on it. It isn't a gimme - but take a look at it. There are DLP models available as well that will do a great job.
Now, you do want to watch ambient light in the room and consider glare and things like that, but for the money, RPTVs are hard to beat.