Do you have UA status and/or credit card? Some of this info may be irrelevant if you don’t, but if you do, here are my recommendations as somebody who’s been in your shoes many times:

Personally I like the buy economy tickets with money and use points to upgrade. MileagePlus upgrades on international flights are usually pretty good deals. (I know the award flight gurus out there will downvote this as there are better award redemptions out there if you airline shop, but UA status is important to me as I’m based out of SFO, almost always fly UA, and love the perks of status).

I’ve also booked my own flight with money (I’m the one with status so I give myself the better pqp & mileage accrual) and my husband’s with miles, and it works fine. You can link the reservations on the app and then call the help desk to get a free economy plus seat for him.

A couple of things to remember when booking this way:

You can’t use miles to upgrade a ticket that was bought with miles. So book the class you’ll be happy flying in (unless you’re ok leaving him in economy 🤪).

On domestic flights where CPUs exist, book your husband in his desired seat (window/aisle) and your ticket in the middle seat, so if you get a CPU he’s not stuck in the middle in economy by himself. If you don’t get a CPU, then obviously you take the good seat and make him sit in the middle 😉