What kind of weapon did this critter has? If 2H then it shouldn't be able to use it and in your case it is a bug but if it was 1H then everything is ok.Just for example, we farm avengers from Enclave with our snipers. We break one arm of the guys with avengers/gatling and two arms of the guys with plasma pistols.
You're telling that NPC only "equipped" it, and yes it's possible. But if he "shoots" after that, then it's clearly a bug.