I was in a similar situation, OP. Had been with my OH for about as long, there had been odd occasions of abuse, physical when drunk, but once he met his new 'friend' everything changed.
I wasn't at all comfortable with the friendship and only mentioned it on the odd occasion (it wasn't worth the drama if I ever stood up for myself) and I never once stopped him from doing anything at all but I was the controlling one 
The abuse was ramped up tenfold, I was blamed for everything that was wrong in his life (that I now know were his own failings), he was walking on eggshells, I was mad, etc. When they became more than friends it got even worse.
I was going through hell in the last few years of the relationship, could see some of the obvious abuse, but it wasn't until I had help from Women's Aid that the full extent of what he'd been doing to me became apparent. I was shocked that I hadn't realised but was very much in the FOG.
Please be very careful, don't try to fix him, do whatever you need to to protect yourself and keep (inwardly) strong 