If she means a lot to you, come up with a language that IS appropriate that you can both use. A way to break a habit is by replacing it with something else.

You think she's a complainer and that's how you feel. When you're both at baseline, talk with her about this. Maybe saying "complainer" isn't going to get her to listen.. but how about saying something like, "Sometimes I feel like there's a lot of negativity in what you say and it brings me down."... or something rather.

Does she complain a lot? Maybe she needs to address something, too.