She's not his grammy...?

My husbands father married a girl who is younger than my husband, (she's 27), and she keeps on calling herself grammy around my 18 m/o son. It drives me crazy. My husband's mother died just 5 years ago, and he's still very upset about it. His new "step-mother" doesn't make any effort to see my son, any time she's seen him it was on holidays or when I invited her over. My husband doesn't want me to make a seen, but I can tell it really bothers him. What should I say?