[Lestat]

You're both right and it depends on the opponent.

99 is vulnerable and you don't want to give a free card if at all possible. On the other hand...

If check/raising the turn is such a standard play with almost any hand as you say, then I'd bet the turn to induce a c/r. If check/raising a worst hand is just a decent possibility, then I'd check behind (as you advocated) and pay off the river.

I'm certainly not gonna criticize your roommate's play. In many instances he would've played it perfectly. Or he might've done better to check, or call down. Depends on the player.