Hi, you can use DotProduct Dot Product | Unreal Engine Documentation between the normalized forward vector of the player (GetActorForwardVector) and the normalized vector from the player to the NPC (Normalize(LocationNPC - LocationPlayer)).
This here will give you the angle between the forward vector of the player and the vector from the player to the NPC
Also the dot product will return 1, if the angle between both vectors is 0 degree, -1 if the angle is 180 degree, 0 if the angle is 90 degree and something in between if the angle is something in between. If you want the exact angle, then you can use arccos of the dot product which will give you the exact angle between both vectors.