2/5 NL Live Game, why did he make this play?

[runnerunner]

He bet $400 into a $165 pot and all he needs is a 5 for a straight. I would insta-muck. If he makes this play without a 5 in his hand, you'll have better spots to stack him later.

Oh, and JAM THE FLOP, there are way too many turn cards that kill your action or could cause you to make a mistake. Any spade 5, 6 or 8. That's 18 cards that you don't want to see.