How do you pay for a divorce on low SSI income?

I have a friend who is on ssi. He and his wife have been seporated for 2 years and live in seperate states with a child living with the mother. I know they base it on both the wife and husband's income to determine the fee of the divorce. But for one, why make only one pay the divorce fee? and two, how can someone on a small measely ssi income afford the court fees and divorce fees alone?