Assuming the player needs permission to re enter, stop play unless an advantage can be played. (In case of advantage I would say it should be a very clear advantage and definitely at or near opponents PA).
If play stopped YC for entering the field of play without permission and restart with a DFK (to the opponents) where the interference occurred. (Law3)
If advantage played, YC at next natural stoppage and restart according to whatever the stoppage was for.
Circs are a little vague, but I think this covers it, though as always the opinions of learned members would be appreciated.
(I saw a similar incident a few years ago - it kicked off big style when the DFK resulted in a goal being scored directly from the restart).