[Surf]

unless you have reason to think otherwise (ie, he's super-passive) CO would have found a raise on the flop or turn with a 7. BB's flop donk certainly could be, but also could be a flush draw or A-hi or a 5 or PP. Also, BB *should* bet the river if he has a 7 b/c c/r'ing faces the 3rd player with 2c and rarely nets him any extra bets at the risk of it checking behind.

Meh, all that said, I'd bet it, unless CO was passivish postflop or BB was overly tricky. As others have noted A-hi, 5s, and PPs call all day. Sometimes you get calls from horrible stuff like KJ, and you need to give them a chance to make that mistake.

Surf