Relationship transitions work as long as everyone is on board with it. In other words, if she is ok with you moving out and keeping a relationship with just him, then it shouldn't be a problem. If she gets upset, and seeing that he is okay with status quo at your expense, then I suspect that the chances of it working out are slim.
Me: 48 - Married, straight, male
Shiela: My wife.
Suzanne: My FWB
Adam: Shiela's LDR