It really isn't a good idea to 1)assume that your friend is gonna help you with YOUR mortgage and 2) Go into a landlord/tenant relationship with your friends without signing a lease. It is just asking for trouble, especially when you are just deciding a couple months in to do this.
Even though we were engaged when we moved in, my fiance and I still got together to organize who pays how much for rent/utilities. Then again we are also both functioning adults. Then again I force him to watch this stuff, as well as, vanderpump rules so he might get rid of me sooner.:)