。
Don't nitpick your friends.
Literal
Friend [possessive-の] fault-finding [object-を] do-[prohibitive-な].
あらさがし (粗探し) literally means 'searching for roughness/flaws' — a vivid compound for habitual fault-finding. The sentence-final な is the blunt negative imperative, distinct from the softening sentence-final な. するな is a direct, no-nonsense prohibition.