Have a one year league bowler on my team who was fitted last year for a new ball.
Finger tip and thumb slug.

If his span is to big what type of results would we be seeing from that?

Based on the Wiki fit I feel it is 3/8" to long as he only gets 3/4's of his fingers tips into the ball. He's been throwing a slight back up.

Is the span hurting his chances to learn how to throw a slight hook?

Thanks

While your right about being fitted while wearing a wrist device and if it's a adjustable one you should have it in the position you'll be using it most in.

Your not changing the span to make room for the strap.

It's do to the position the device holds your hand. Like the old Gladiator glove, It held your hand pretty flat and rigid.