Kyle Cooke got hit with a brutal one-two punch -- his ex-wife moved on with his best friend ... and TMZ has learned he feels completely blindsided.

Sources familiar with the situation tell TMZ ... this isn't just about Amanda Batula's new romance with West Wilson -- it's the fact West was one of Kyle's closest confidants, making the situation feel like a straight-up betrayal on two fronts. We're told the "bro code" violation has hit especially hard, and Kyle is not taking it well.

As we previously reported ... Kyle had been hearing rumors for months about Amanda and West possibly being a thing, but when he confronted both of them directly, multiple times, they denied anything was going on. Even as recently as this past weekend, our sources told us Kyle believed it was all just speculation.

Now, with Amanda and West finally going public in a joint Instagram statement Tuesday, the truth has landed all at once, and our sources say the emotional impact has been intense.

We're told Kyle is very affected by the situation ... not just because of the romantic fallout, but because someone he trusted deeply went behind his back. The shock, we're told, is a double whammy.

Adding to it all ... Kyle and Amanda were together for a decade, with much of their relationship playing out on Bravo's "Summer House," making the split and this new development even more personal and public.