It's tricky, because sometimes adults getting involved in their children's arguments can sometimes carry on long after the kids have sorted it.
That's the tack to take with this mum - you need her co-operation, so an 'attack' will simply make her defensive by instinct.
Try the 'we're mums together, so we can sort this between ourselves ...' line.
Be calm and polite but firm - maybe her son is not aware that his teasing is causing upset, so his mum can have a quiet word to start with.
If you get no joy with them mum - or the picking carries on, i would have a word with your daughter's teacher and explain the situation, and your action thus far. Schools have plenty of experience with this kind of situation, and can probably sort things out from their side.
Good luck.