Truthfully, providing that she is otherwise a healthy mare, I see no reason to not start working with her now trying to get her back into shape. Of course, for the sake of the foal, keep the sessions short. 10-15 minutes should cause no problems at all.
The biggest issue you are likely going to run into is having to separate her from her foal while you are working her. If you try to work her with the foal in the pen or even nearby, then she'll be completely distracted by her foal and if you are trying to lunge or long-line her, the foal could get tangled and that would be disastrous.
Even if they are separated where they cannot hear/see each other, you are likely going to have problems with keeping her attention. BUT, that is an excellent reason to go ahead and do it. It will be a learning experience for her.