Can you tell me what could I tell a loved one that will make them quit drinking diet sodas?

Don't. Please forgive my frankness. Criticism erodes love. You've found love, but I would not tolerate someone nagging me about food choices. After 30 years, I've concluded that the scares about aspartame, etc. that you'll read online are bogus -- in fact, I'd suggest you ask your local junior college chemistry prof. Diet soda is far healthier than other bellywash and even high-calorie fruit juice.