I don't even know what the dope meant by "ring seizure".
Either the piston seized, or it didn't.
A "ring seizure" could only mean that the rings rusted to the cylinder and then were broken free when ridden. Then it started smoking because oil was leaking through the porosities caused by the rust in the cylinder.
Either way - ANY four stroke engine that seizes has been poorly treated. There's no way around it - the bike had something either done wrong to it, or was wrong from the factory. And in either case, the damage would probably not be limited to the piston/rings.
This is not a "common problem" on the '01 XR. It's not a 'common problem' with any modern four stroke that I know of. Also, as you mentioned - there have been ABSOUTELY no engine changes on the XR250 since they dry-sumped it in '96.
If the rest of the story is true - about the owner trading it in on a CR - I'd think the scenario went something like this.
1. Pick up new XR at dealer.
2. Take it to the MX track.
3. Scream the guts out of the poor thing for a couple of hours in a hopeless attempt to keep up with buddie's CR.
4. Return it to the dealer because it 'runs funny' and is smoking lots.
The dealer is a dummy - or thinks you are. Either way, I don't know that I'd want to purchase from him.