I'm not sure there is so much you can do to actually "help", or at least not
in the way you mean, depression takes time, sometimes medication and a lot
of support to recover from, you can ease that path, but you can't solve the
problem.

I have a dear friend with severe, recurrent depression that meds to not
always manage to control, over the years, we've learnt that there is nothing
we can do to prevent each episode happening, we can usually spot it earlier
than her and get her to the doctors, we can slow the onset a little, we can
support through it, but we can't do much more, the rest has to come from the
doctors, counsellors etc.

Unfortunately stopping breastfeeding is known to have an effect on postnatal
depression, rates are higher in mothers that don't breastfeed and a low
patch even if it's isn't depression is commonly reported after weaning, you
probably don't want to tell her this right now! But if she has unresolved
issues in this area, they are important to talk through and if she mentions
trying to start breastfeeding again (you don't mention how long since she
stopped), then I would encourage her.

Cheers

Anne