As much as it will suck to watch, I'd step back and let him do his thing. He may be offended and think you feel he's "too fragile" to handle this girl and that might be another blow to his already low self esteem. They sound like oil and water anyway, their personalities. The real her will rear its ugly head for him eventually.
Some guys/girls feel the best way to get over someone is to get under someone, literally. Maybe he needs a new piece of @ss to get his mind off the ex.
So, so you think you can tell Heaven from Hell,
blue skies from pain.
Can you tell a green field from a cold steel rail?
A smile from a veil?
Do you think you can tell?
And did they get you to trade your heroes for ghosts?
Hot ashes for trees?
Hot air for a cool breeze?
Cold comfort for change?
And did you exchange a walk on part in the war for a lead role in a cage?