As for your specific situation, a couple of questions first...
1) Was the enemy aware of the party(and Monk) prior to combat?
2) Was the Monk visible to the enemy as he ran towards them (i.e how did the Monk get behind them)?
Basically, it would depend upon the specifics of the situation. If the enemy is not aware of the Monk at the time of his approach, then his chances of sneaking up behind them shouldn't be too bad (see below for caveats).
However, if they were aware of him, and then he ran around behind them, you basically cannot sneak up on somebody who knows you are there (especially in a hostile melee where there is constant motion, just because we do not choreograph every step, that does not mean that combat is not filled with back and forth, twisting and turning. Simply look at the fight scenes from ANY movie regarding this, so there is always a chance of being spotted and it takes nothing to yell at friend, "Behind you!"
And don't forget the Common Modifiers table -- in melee is +10; surrounded by melee is +5. Under the most ideal conditions, I would say that the TN is increase by +5.
Also, Stealth is basically a movement action. You make the roll, and you have moved 5' for each AP spent making the movement.
And perhaps some FAQ entries might be a good idea... just have to figure out how to phrase things properly... (not to mention have the time available)