“But I think it’s a slightly odd thing to say”
This is just a guess, but Sarah screaming about her son being a vet was probably a misguided, but failed attempt to appeal to any shred of patriotism and decency these thugs attacking her son just might have.
IF she really said that, it was a pretty dumb thing to say. If some guys getting pounded, I highly doubt the guys doing the pounding are going to stop just because the guy used to be in the Army.