I think you are maybe trying to be defensive on your son's behalf, which he obviously doesn't need, and may not actually want.
Two of my three sons-in-law have full heads of hair and tease me mercilessly about my being folically challenegd, which is fine, it's the relationship we have, no-one takes offence.
If your son was hurt by this girl's comments, it's for them to sort out - do not get in between them on this, they will band together and you will miss out.
Not everyone behaves in the ways we would like, or the ways we behave ourselves, you just have to bite the bullet and get used to it. i am sure this girl does not mean any disrespect - but if it really bothers you, ask your son to have a word with her about dialling it down a little.
Do not confront her, or be personal towards her, you will cause a rift, that that is no good for anyone.