Quoting Mj's Crazy Mama♥:" It's not her MIL/or her Mom that has a problem with it. It's HER Mom's friend that has a problem with it."
Ah, I got lost in it. Why does her mom's opinion matter?
All my BILs and SIL call my mom mom. I wanted to call my MIL mom, she was the most amazing woman. I loved her. She told some nurses in the hospital that I was the daughter she never had (she had 6 sons). But, it's not something you do in this country.
My mom calls my dad's mom 'mom' too.
If anyone has issues with it, roll your eyes at them and ignore them, or tell them to shut up, or just laugh at them and walk away.