Foul ball bounces back to fair

[sketch]

At my son's little league game yesterday, we were on the field with the other team batting. The batter hit the ball which rolled down the first base line clearly in foul grounds the whole way. Then something strange happened. The ball hit the edge of the grass and bounced back onto the field before it got to first base. Our team saw it as field interferrence and the other team's coaches insisted that it was fair ball.

My question is simple: Is if fair? The teenage ump was not sure, but ultimately called it fair.