Try and have a little one to one chat with her. Let her know gently that you're aware of what's going on and that you're going to make it better. Try and get some details out of her and then i'd start with your DD's teacher. Don't push too hard as she may just bottle things up.
Good on her friend for telling her Mum. It means she's got someone to support her.
The first thing i did when i found out my DD was being bullied (emotionally not physically) was phone the mums of her 3 closet friends to ask them to get their daughters to look out for her. Making sure she wasn't alone in the playground so that the bullies couldn't get to her.
I had a meeting with DD's teacher who immediately made the deputy head and all playground staff aware of what was going on (head was away at the time). The class & then the entire school were given general lessons on friendships, then DD had her own chat with the teacher. The teacher set up a worry box in class so she could leave a note rather than going up and talkign to her..
Finally the bullies were confronted but annoyingly because it was a first offence (despite going on for several weeks) we were told their parents would not be informed.
A more violent verbal threat occured the following week, at which time we went straight to the headmaster.
It was a heartbreaking time for me (& DD). The anxious wait for her to come home from school, trying to read her face to see if she'd had a good or a bad day and seeing her so sad.
I have to say that after the summer holidays, it was like it had never happened. One of the girls subsequently left the school but we never did find out why.
Anyway hang in their, be patient with her and keep a note of anything that happens.