He is a legal receiver. If there had been an interception and then the receiver, inadvertently, stepped on the side line, there is no foul as there had been a change of possession. The same is true for a kick. If there had been a change of possession, he is guilty of nothing as long as he returns to the field at the earliest possible time, and onto the field directly. He cannot run 10 yards out of bounds and then return.
Illegal participation causes a great deal of consternation and question.
If he stays out of bounds and does not return for the play, he has NOT fouled. It's just a rule that takes a little more to understand. There is no loss of down provision here. Also, if a receiver, accidently steps on the side line and returns, use judgement if the situation has no effect on the play at hand. For example, don't call IP if the foul occurred entirely away from the action at hand.