^^Wise words from a wise woman.
^^
Look, in my opinion and it's certainly not the definitive one, love is what you make of it. What I mean is that anyone can be absolutely miserable in a relationship if they try hard enough. The new girl is hot. She's an unknown. She's exciting. She's a mystery that wants to be solved. The old girl is still pretty hot, but he's discovered most of her secrets and explored the mysterious unknowns, so, she's a bit boring.
Personally, I'd take boring any old day of the week. Exciting and mysterious is good, but comfortable and familiar is so much better, in my opinion.
I'd say get the old boy to settle down a bit and take a little time. I'd tell him to not tell the old of the new and keep the new at arm's length for a while until he's sure he's sorted all this mess out. Only then can he objectively look at the situation and come to the 'right' conclusion, whatever that is.
Cooler heads prevail, right?