Doesn't matter for now, but for the future I feel it should be noted that by RAW, Corren gets his sneak attack damage any time an ally is also adjacent to the enemy.
The idea is, regardless of who is specifically engaging whom, a creature threatened by multiple opponents has its attention divided enough that a canny individual can spot an opening.
Of course, he doesn't get advantage on the attack roll like he would if he had been sneaking.
Edited: daishain on 9th Apr, 2017 - 1:32am
When using miniatures on a grid I would agree with the RAW for the most part although I would disagree with the idea that simply because they are adjacent the bonus should apply. I'm dissenting from RAW in this regard because to me, its not just that an enemy needs to be adjacent to make the bonus apply but an ally must have done something to draw the targets attention (Attacked them, taunted them, etc.) so an ally could be adjacent to an enemy but that ally is actually fighting another enemy and not doing anything to really distract the target so the bonus would not apply. In this case we are dealing with theater of the mind over a grid anyway so those kinds of tactics don't really apply the same way.
The rats that are attacking Corren are not concerned with, or even overtly aware of Leo or the others at the moment as their attention is on Corren so their attention is not divided to the point where Corren could take advantage of it even though Leo is close by, perhaps even within five feet of Corren. Leo may be "Adjacent" but nothing he is doing or has done has drawn the attention of the rats attacking Corren.