Can someone agree to not pay child support even tho they are receiving help from the government?

My boyfriend has a daughter that he didn't know was his until recently. The grandmother has been raising her since her mother is not a good person. The mother has two other children that live with her by the way. The grandmother asked my boyfriend to sign over guardianship to her since the mother would not. He did, and the grandmother said she would not ask for any money from him, she just wanted his daughter to get to know him. She then asked for assistance through DES for medical insurance and day care. Then he received a court summons to establish child support for next week. My question is, can our original agreement still hold up where the grandma said she did not want child support? Can they agree on the amount of child support out of court and tell the judge they already settled the matter at the hearing? Can he then ask for custody of the child although he agreed to have the grandma be the guardian? Can the child support be split between him and the mother of the child even though she does not have a job? I just want to know what his options are because this came out of nowhere and she agreed to no child support and needed his help in her obtaining guardianship. Thank YOU!!!! Oh, and we live in Phoenix, AZ and they live in Tucson, AZ.