I think, based on images, like when the MC was drunk at her place and he seen what appeared to be a military man, that he was most likely in the military and killed in action.
Notice that (if you played the Bella path anyhow) Bella mentioned to Jill how she felt guilty about being with someone else as her explanation as to why she never said anything to Jill about being with the MC, and how Jill never raised the idea of Bella cheating or being married, but understood her feelings.
Bella wanting her husband back makes sense if he was killed in action. I have found myself saying something similar about my late wife.
All indication SEEM to point to him being dead or missing in action (same deal as far as I am concerned). Bella certainly is NOT an adulterer! Man alive, she's been without any men in her life for three years and it's taken all of her will to do anything with the MC, and even then she feels guilty and said so to Jill (if you didn't play Bella's path, than you never read the dialog when she explains how she felt and why she never said anything to Jill).