I would tell her that winter is coming around, and as well as keeping herself warm, she should also keep her friend warm. Tell her his color will pop, er, something. Make it more about keeping her happy (even though the Betta is VERY important) she is your boss, and you don't want to screw your job up. Or, if you have an extra tank laying around, bring it in to her as a present. It's a tough situation.
The days in my life that make me feel the best are days where I am able to bring home a Betta and rescue him from an untimely death.