The problem is solved. Now my friend’s Sea Vernom flies like a dream!
Tailplane/Stab was placed too high in front, after having lowered the tailplane at front approx. 1.2 cm the plane flies like a dream with no bad tendencies to tipstall or similar, it flies like a boomer
Took some pictures along the way, the pictures tell it all. Thank you all for your help and excuse the late update on the thread.