OK - In that case you cannot refreeze it. In that case you have several options... As Dave mentioned you could cook and eat it yourself and buy another one to give to him. You could also hot smoke it and freeze it - however this is not so good once it has thawed. I would not suggest doing anything with Nitrite though as this is overkill for fish and you may as well just prepare it as if you were cold smoking.
What I would do is to cold smoke it instead. If you have never done this before then BE VERY CAREFUL as you will not want to eat plain hot smoked fish again !!!
For speed simply
- Fillet the fish and pin bone it.
- Make a cure mix of 50% salt and 50% caster sugar. Mix in some fresh shopped dill of fennel leaves if you want.
- Liberally coat both sides of the fish with the cure and allow it to cure for 24 hours on a wire rack in the fridge.
- Wash the fish under a running tap with cool to lukewarm water to remove all of the cure. Pat dry with a paper towel.
- Cold smoke for 24 hours between room temperature and about 28 C (80 F)
This will give you an idea of what you are looking to achieve...
The thing to remember is that here you are providing bacterial control by increasing the salinity in the fish, reducing available water and also supplying smoke - which is a mild antibacterial agent. The curing increases the salinity and the smoking at that temperature accelerates the water loss from the fish and provides the antibacterial smoke.
Once you have got it to this stage then you are looking at (commercially) 10 days of chilled shelf life. You coule either slice it and give it to him as traditional sliced smoke salmon or cut it into steaks, vac pack and freeze.
If you decide to freeze then on some of the fillets you may want to first add a little sliced fresh ginger, a ring of thinly sliced lemon and a thin slice of chilli. When he wants to eat it then he can simply boil it in the bag to get a wonderful fragrant dish. (12 minutes from thawed or 15 minutes from frozen)
Remember though that this is fully cured smoked salmon and could be thawed, sliced thinly and eaten raw.
Photo of Woodchurch Smokery Salmon curtesy of SlimJimUK