Quote Originally Posted by net surfer
Since she started calling him ты, I think it's OK to ask.
Besides I think if you've been working/talking to a person for pretty long time and you feel you're equal, you can offer to switch to ты and it'll be OK.
It was fine for her to address him на "ты" and accept the fact that he addresses her на "вы". Their relationship is established for now and any change in addressing will change it.