A co-worker is interested in getting into triathalons and cyclocross. While he could go out and buy a brand-name cross bike, he very wisely has decided to first find something inexpensive to see if he'll stick with cycling. (He already runs and swims.) Getting a more expensive bike can come later. He came to me because he knows I do a lot of riding, but my knowledge is limited to touring and commuting, mostly.
His budget is five bills, but he knows full well that finding a cross bike for $500 is nigh impossible - I gather it'd be tough to even find one used for that little. Are there other types of bikes that would do for him? Of course, if he gets into serious competitive events, he'll want to get a proper cross bike later on. (I gather bikes for triatholons and CX are pretty similar.)
Would my co-worker be relatively happy with, say, a light hardtail mountain bike or a hybrid? He plans on doing century rides in the short term. Or is he better off looking for an aluminum road bike with slightly wider tires?
Anyone here ride tris or CX on a bike that isn't meant for it?
Edit: Thanks for the answers, these are helpful. I'd like to add that my co-worker knows very little about bikes, and building one up is not an option for him, although fixing up a used one might be.