Yes, that fin would destabilize yaw somewhat, but most helos have lots of yaw stability (because the tail rotor behaves like a piece of fin. It produces thrust when yawed that restores the trim. In fact, the tail rotor produces about 4 to 8 times the thrust of an equivilent vertical fin!
More importantly that big forward fin would add dihedral, which is the roll you get when you yaw. IFR handling requirements state that when you put in some pedal to yaw, you must have to put in some opposite roll cyclic to counter the roll that the aircraft naturally has. In other words, some right pedal to produce a few degrees of yaw must produce some right rolling attitude change. This right roll must therefore require the pilot to put in some left cyclic to trim the aircraft level.
That big fin above the roll center would create a roll force above the roll centerline that would create a right roll when a right yaw is put in.
The reason it is there is probably to allow the minimum IFR speed to be reduced somewhat, since the natural dihedral stability of most helicopters vanishes at about 60 to 70 knots. What is Vmin IFR in that 412's flight manual?