I agree that you'd be better off with lower dihedral but I disagree with the shop owner's comment.

I've seen two PT-40s with the high dihedral wing. One had functional ailerons and the other had them locked. Both were very stable and exhibited a LOT of self leveling characteristics. The instructors who flew them said they had to fight the plane more than they liked to keep it banked in a turn. Neither plane could take off or land well with any crosswind at all, even with an instructor in control. The slightest crosswind would blow the plane over.

I built a PT-60 with the low dihedral wing. It was very stable and handled crosswinds fairly well. It did not exhibit the same self leveling characteristics that the high dihedral PT-40s did. At the end of a turn it needed a little opposite aileron to level the wings.

Since you wrote in another post that you've already soloed with your dad's plane, I wouldn't attempt to modify the wing. I'd repair the damage and fly it as is while building that second plane.