I don't know what your d-in-law has, but it's a very typical doc cop-out to blame it on PPD. She needs to find a completely new and different - and competent - doc and begin from scratch. An anti-depressant might help in the short term, but she needs a definitive diagnosis and needs to get her life back, for herself, your son, and mostly, her children. Don't let her be snowed by the typical "You're overstressed" doc remarks. That doesn't always apply. Good luck to her.