It depends what she means by arms and what you mean by shoulders.
If it is on the shoulder blade will that isn’t arm.
If however it anywhere on the sleeve of a shirt type area, from the seam around the armpit and above the shoulder, that would be the arm.
If you are asked was that anything else distinctive about him, and she said about the body hair, that would certainly assist her and not him and assist him and not her if she said there was nothing else.
It depends on the circumstances as to why she didn’t mention it.
It also begs the question as to why, if she has had all these other men, why she is singling this one out for “special attention”.
If she is for example accusing this one of rape, it can go against the accused because if she has had so many many willingly, it seems unlikely that she would single one out to alleged rape if it didn’t happen.
The really not enough facts here to do anything other than speculate.