My good friend has been in a relationship with a single mother for about 8 months now and basically was suckered into it but didnt mind cuz of the easy access to sex. She just got his name tatted on her lol and made him delete all the chicken off his social media

What a joke... but wait,

Hes about to move into a half a mil house he bought on his own and has a stable job. He was going to let some other buddies move in to help pay the mortgage but now shes convinced him to let her move in with her kid and "pay rent". What advice can I give him to not get fucked over as he will.... list her as a tenant? Make her pay rent through cheque?