Got video of it? IG is inherently a judgement call; to a smaller degree, holding is as well. I can't speak to if they really walked off 15, as it should be 5 from the spot of the foul and loss of down.
Where was the LOS?
and where (what yard line) was the QB when he threw the ball?
What position (1-5) was the ball at the snap? (1-5)? (helps determine tackle box on the field after the snap and kids start moving)
What yard line was the QB at when he threw the ball?
Where was the QB in relation to the tackle box when he threw the ball?
Did the ball reach the LOS?
Was there a receiver in the area?
All those questions are important and each brings a vital piece of information to the table to determine whether or not there was an IG foul, and if the referee thought there was, where it would be enforced from.