Good question...I'd first ask you a couple of questions..How long ago did she have her child? How long has she been on anti-depressants?
Many of the anti-depressants have a side effect of weight gain. If she is taking the meds for a post-partum depression issue, she may not have been on them long enough for this side effect to set in. If it did...or if she continues to have a hard time dropping weight she should consult with a dr. and see about switching anti-depressants.
On a side note, as I'm sure you're both well aware, one of the best ways for women to cut weight after childbirth is breast feeding in addition to exercise.
On another side note...my wife is on an anti-depressant and has a hard time dropping weight. Right now she is on a Hyperdrive/Meltdown stack is has started to drop some weight. She's down 12 lbs since the start of the summer. She hits it pretty hard in the gym as well as doing sprint work first thing in the morning.