I'm not big on watches, but I move in circles that are, and unless he's upgrading to a more expensive watch, he'd be crazy to sell his if he gets enjoyment out of it. It's a Patek Phillippe.
For about four years, I had a beautiful car that I couldn't drive for medical reasons. DP did drive it, daily, but would have preferred to trade it in for a more practical car. I loved my coupé. I would have supported him getting a more practical car, and we have one now, but I didn't want to get rid of my coupé and it wasn't bought with "family" money, it was mine, so while I listened to his opinion, it was my decision.
We have traded the car in now, partly because the engine went, and partly because we compromised and now have a better car that we both love. I got to choose the bits I loved too, and whilst I miss my coupé, the new car is newer and better and more suited to us. It's more practical, too.
In your DP's case, you are suggesting getting rid of watch that he will be attached to, that he wears every day, to build an extension. It's just not comparable. And if you can't fund an extension in any other way, you're unlikely to be able to buy him a better Patek Philippe.
I'd try my best to stop seeing it as potential money, and leave it.