First, congrats to her!
Second, breastfeeding is a great way to burn extra cals, and of course the BEST food for the baby. So hopefully she will continue to BF (some women say that BF'ing makes them "hold on" to weight, but I feel that is incorrect. So don't believe it).
She's only 6 weeks post-partum. I think she really needs to just be a little patient with herself. If she's less than 20 pounds from her pre-pg weight, that's really pretty good. She should aim for about 1-lb per week weight loss. She can count calories, but just not go too low, since she is nursing, and you need about 350 calories per day to make milk. So I would personally think like 1900 per day or something would help - I'm not a doctor, so this is just from personal experience.