Only if he believes the price won't tank more than 2-fold until he gets the money. Also he has now a much higher risk that someone locks up 2 BTC to lock up 3 of his. Potentially forever.
Would you sell me 1 BTC right now via bank transfer if we both need to lock up 50 BTC for that?
If we are exchanging 1 BTC for $650 and we lock up 1 BTC or less as a collateral, then I can receive your $650 and let my 1 BTC stuck as a collateral forever. I'm not losing anything (I'd pay 1 BTC anyway), but you lose 1 BTC + $650.
I'd consider a collateral that is big enough to not cheat, but not too big to go into a different category. In absence of mutually trusted third party (e.g. online exchange), the only way to resolve the issue is to "buy" the trust by agreeing on some big enough collateral.