Here's some suggestions veteran pilot Chris Brewer (GSAL) gave on this plane, and it's tendencies...

"I would first check the incidence of the wings and then check for warps. It's not difficult to glue the wings together misaligned. I've owned a couple of these and they all flew very well. My most recent one came with warped wings. I had 3 degrees of washout in the right tip and 1.5 degrees of wash-in in the left. Every time I pulled a "harrier" or "wall" it'd snap left. I set the wing tips at 0 degrees relative to the root by twisting them, and then shrinking out the wrinkles with the heat gun. It took 6 or 7 times with each panel but it was well worth it.
You should also check that your elevator halves are moving symmetrically. They'll produce a roll if one side is moving faster/further than the other.
I don't think that it's the CG, I think that there's something going on with the wing. My plane is 12 lb. with 45+ degrees of deflection on the elevator, balances at 45% of the chord and doesn't snap..."

and he further added:

"You
may want to tell him that a perfectly straight wing is the most critical issue
here but that the lighter the plane is the less severe the snap will be. My
flying buddy had a H9 with a Moki 1.8 (mine's a 2.1) that was a pound lighter,
crooked as hell but flew just as well! Your guy could add 1/2 degree of
washout and never worry about snap/spin's. The only downside would be a mushy
snaproll..."

Hope this helps....