If it helps, tell her I've been to emergency counseling (as in I called and they said if I didn't carry myself to the center RIGHT NOW the deputies were going to come pick me up) because I was suicidal and did not want to lose custody of my son. They were even understanding enough to allow me to contract for safety and go home instead of being institutionalized because I was concerned that his father would use institutionalization against me in court. They even let me bring him to appointments when it was either that or not go and they found someone to watch him for me while I was talking with the therapist.
If she has someone at home who can vouch for her safety and keep an eye on her child, there's no reason for him (or her) to be taken away. If she does need to go somewhere, hopefully she has you or other family she can depend on to watch her child so that he doesn't get put into the system, which would absolutely not be best for anyone. It's a big and reasonable fear for a lot of single parents, and it's definitely something that often acts as a barrier to us acting for help with mental illness.
Many doctors or drug companies can help with lower cost prescriptions. Unfortunately I don't see clonazepam or quetiapine on the $4 list for wal mart. There are a number of different medications for depression, though. Perhaps she could work with her doctor to find one that is affordable and works for her.