I agree with Ms. Shapen! Your friend will make the changes when she is ready, and if she chooses to keep her diet drinks a part of her life, so be it. That may just be one thing that's worth keeping to her.
When I first started WW, I drank a lot of diet coke. Mainly because I had been addicted to 'real' coke, and diet was the best way for me to curb those coke cravings. I can tell you, as probably most diet soda drinkers can "WE'VE HEARD IT ALL!" lol... really, we have.
It comes down to letting every dieter walk their own path at their own pace.
One of my friends used to constantly give me a hard time about the diet drinks I consumed. She would see me order diet coke while we went out to eat. What she didn't realize was that I was using it as a tool... not only did it take the place of a calorie packed beverage, but as I got full, I'd take sips of my drink to keep myself (and my hands) busy instead of picking at the remains of my plate.
And I knew it was a temporary tool. One to be used while I got used to eatting less. As I got used to a smaller food intake, I gradually cut back on the diet sodas too.
Now I just grab a cold diet coke if I'm hit with a really strong craving for junk food, or if I'm in a poor mood and ready to stuff my face.

That's just what works for me.
I guess the diet soda issue is just a sore spot with me because that particular friend gave me a really hard time about it. It was tough for me to take from her, because I was SO HAPPY to be losing weight, but ever time she saw me drink a diet soda she harped on my unhealthy choice.
The worst part was, she was currently gaining weight as I was losing weight... and with every diet drink I ordered, she gulped down the same amount of sweetened lemonade (or chocolate shake!)... But I didn't have the heart to point out the flaws in her eatting habits, so I just listened to her comments in silent frustration. Eventually when I couldn't hear it any longer, I told her I was using it as a tool to stop drinking empty calories. She looked down at her lemonade & got a little flushed... she's never mentioned it since. =/
Make mention of it to your friend if you still feel the need, but just be gentle. I wouldn't want her to be hurt and angry the way I was.