Choose to cut him off, or forgive him
Your boy made a conscious choice to put his sex drive (or feelings, both are equally despicable) over your long-term-bromance. Now it's up to you to make a choice: Will you forgive his faux-pas, or cut him out entirely?
If you choose to forgive, know you'll probably never feel entirely comfortable introducing him to your new boo. In the back of your mind, he'll always be circling her like a starved hyena, waiting for you to walk away so he can devour your relationship's remains.
Or, you can be the bigger man and win some karma points by letting his admittedly major slip-up slide. You can't claim ownership over a woman like she's a cashmere cardigan, after all. If he chooses to wear your well-worn sweater with the freshly-made pit stains, god speed!