I'm not so sure you should just ignore it. I think it's rude of your SS's girlfriend to comment negatively about your husband's clothing style. Period. No matter what he wears.
The first time one of my DDs' boyfriends makes a comment about my clothing other than a Eddie Haskell (from "Leave it to Beaver" tv show) type comment of "Gee, you look nice Mrs. Balletmom!" will be the last.
I see nothing wrong with a polite, humorous, and firm response along the lines of, "Gee, we really weren't looking for fashion updates from SS's girlfriends. One of the joys of getting older is dressing to please just yourself."
If it were me, I'd probably add, "Be thankful I'm not wearing a muumuu and house shoes, cause I have earned the right to be comfortable!"
I would not have dared to tell any of my boyfriends' fathers how to dress, or that matter, my FIL.