I have physical custody of my son. My ex wife receives SSI payments, and my son receives payments from her SSI claim. Is she still legally obligated to pay child support?

She's legally obligated to pay or not pay based on what was ordered in the custody hearing.
If her financial situation has changed, go back to court.

She was ordered by the court to pay $50 a week, and this was before she was receiving SSI and wasn't working. Now she claims that the SSI should be in lieu of child support, and that it is illegal for my son to be getting both.