Unique problem?

Fished with a guy yesterday that could cast about as far as anyone
I had ever fished with.

That being said.....

He had the most trout NOT stay on the line I have ever seen in an outing.

The only thing I can think of was there was too much line on the water and when he set.... the sets had no snap to them due to ALL of the line out? No fly penetration?

How would you fix this problem .....other than shorting up the casts.....?

Flies were new and gap was fine.

He lost minimum 20 before he got them anywhere near to under control.