Ex Mac user of 17 years (now Linux) and keyboard enthusiast here.

The crucial difference between Mac and Windows layouts is not whether there's a Fn key to the left of Space, but the order of the Option and Command keys. Mac has Command next to Space, while Windows has Alt (= Option).

If you're anything like me, then your muscle memory (after a couple days or weeks of accommodation) won't give a flying fuck of whether the Fn key is missing. In the long run, it will adapt as you switch back and forth between keyboards.

Your muscle memory, however, may punish you hard for reordering Option and Command. So focus on fixing that. Most newer keyboards have a firmware setting or physical switch that rearrange the scan codes.

If yours has no such setting, MacOS has a per-keyboard software setting that allows you to remap those two keys specifically, but I'd advise against using that because it never worked reliably for me. Instead, email the vendor and ask for advice.

Once you got the scan codes reversed, consider switching the keycaps accordingly to make labels match (if they're the same size. Otherwise, ignore the labels.)

YMMV but maybe it helps.

I play a lot of chess and every once in a while someone gives me a chess set which really pisses me off. I appreciate the thought but I hate adding to the landfill. I drop it off at goodwill. Maybe mention the keys stopped working because the keys were not able to take the great usage you got out of it. That’s it! Call the person up and say: great keyboard but it could not stand the high usage.

Just depends on how much you want to keep it and if it supports QMK style customization.

I use a Keychron Q5 with customizable keycaps. It's connected via bluetooth to both Mac and Windows machines and also only has three keys to the left of the space bar. Personally, I don't think I've ever once used the "Globe"/FN key.

I'd be honest that the problem is that you aren't using a Windows computer. If he still has the receipts, he might be able to replace it with a Mac version, or something else entirely.

You have a technical reason that should not result in hurt feelings.

You can tell him that you gave it to your D&D dungeon master friend who really loved it.

Oh, first off, "Daughter's boyfriend"... You (may) don't like him.

Use it for Linux machines. They'll be delighted to see it "at work" doing nothing in particular.