Thanks for all the valuable input!
Here are the results:
We talked in a friendly manner, two times actually: on Thursday and on Sunday.
We (the rest of the group and I) didn't confront her and tried being gentle. She didn't recognize being disruptive, but we talked her into not voicing her objections until after any given game session. Not the best result, but at least it's a playable consensus.
That being said, we tried to find out if she was uncomfortable or something - after all, she wasn't like that all the way, only the last couple of months. And thanks to everyone, who suggested digging deeper! Although she didn't say it outright, our group is dead sure she isn't comfortable with the rules and her character and that just might be the reason she argues that much. The thing is, her English is much worse than she thinks and she fails to understand some rules. For whatever reason she doesn't acknowledge it and we don't want to hurt her feelings (one other group member doesn't speak English at all, but it is well known and we always help). I guess we'll have to help her without her knowing that. We agreed to spell out every dice pool we form, to reference rules often etc.
She was less disruptive this Sunday, but there are problems still. And to be fair, the phonebook advice proved to be as effective as the negotiation advice: I tried phonebooking her a couple of times and it worked just fine, everyone was happy in the end.
For all the devil's advocates: thanks too

You've made me put on my self-reflecting cap. But
All4BigGuns,
We only ever get one side of the story, and no one in modern society wants to admit that maybe they have the problem. Instead everything is always the other person's fault. Without the other side of the story, we simply can not truly give real and useful advice on the situation.
My aim was not to fix the blame or to prove something. I sought advice on preserving my gaming group and many omae here helped me with their advice, including you.
What color was the tag and the desk? Just where on said desk was it placed? In an open area with no other items near it or next to/behind something larger or more colorful, thus more like to draw the eye from the tag. In the case of the ambush... well I'm sure y'all get the drift. My point being perhaps she's trying to help with legitimate suspension of disbelief issues but has poor communication skills. 
Well, if you really want to know, the tag was black and it was placed in a plant pot with a cactus on the desk. Not the stealthiest way to go, sure, but she really made a scene of it.[/spoiler]