I agree.
Talk directly to her, don't do it over email or the phone.
Suggest to her also, that playing a magic user is very very rules heavy, and, in the past she has not shown that learning the rules is something that she is interested in. Suggest, instead, that she play a strait up fighter, which will let her absorb the basics pretty well. (Or perhaps a fighter/cleric, with more levels in fighter then cleric. This will give her the simplicity of purpose of a fighter, with some spell casting experience on the side. At least, that's what I usually suggest to somebody that has never played before or isn't big on rules. When I first started I was not big on knowing the rules at all. I really liked the Rogue, because I didn't have to worry about spell casting, but I still got cookies like use magic device, backstab, and lockpicking that made me feel special. I absorbed the rules over time. Eventually, I evolved into somebody who plays Bards.)
In other groups I've been in, when two people (who may or may not be involved romanticly) would start to bicker, one of two things would happen...
1. The person running the game would interupt them, and make a ruling about whatever it was that they were arguing about. They would end their ruling with something like "and that's the last I want to hear about it" or "and that's all that needs to be said about that from here on out", and then we would continue.
or
2. Somebody would suggest a break, we'd strech our legs, go to the rest room, get soda, order food, what ever, while the people finished their bicker. This would last no longer then 1 minute, and it cut down on the awkwardness a bit. After a while (not a short while mind you, but eventually) just saying the words, "Let's take a break", was enough to defuse the situation, because the bickerers would realise that they were pissing everybody else off at that point.
In the end, you have to do something. Because your players are not having fun, she's not having fun, and you are not having fun. It's hard, and unpleasent, and you don't want to deal with it, I know. It really sucks that this falls to you, but it does because you're the guy running the game.
You want to be gentle, you want to be nice, that's good, there is nothing wrong with that. But at some point, you're going to have to be firm, that doesn't make you a bad person, and people who are your friends will understand.
Hope that helps.
