While it is common for people in their manic phases to spend too much money it does not excuse them from not being financially responsible to others. Everyone is responsible for their own actions. He should make arrangements for someone to handle his money if he can not do so on his own. I think it is a good idea to have him pay for the rent in advance when he has the money. I hope your son does not get into financial trouble of his own by trying to help this person.