I'd say either agree to call her 'mom' and just never do it - how often do you use someone's name in conversation anyway, and it doesn't sound like you'll be hanging out there every weekend.
Or, agree to it and then call her some ridiculous, obnoxious derivative. Like, call her 'ma-ma' in a baby-ish cadence, or 'moooooooom' all exasperated every time, or 'mommy!' or something stupid. She can't complain, he's basically doing what she wanted, he's not actually calling her mom and he's mocking her. Everybody wins!