Hmm, I note a couple of things in your post worthy of discussion. I've only quoted the above part for a reason, I'll come to that in a moment. The key things here appear to be:
It's a special trip for this gentleman;
Cost;
Finding somewhere interesting to go/do once arriving there.
Apologies if that's not the case, feel free to correct me if I'm wrong. Taking the above points into account, I refer back to the part of the post I've quoted. This is obviously going to be a special trip, a gift, for this gentleman. Whilst I see no mention of why the gentleman would like to try the Caledonian Sleeper (just for the experience I would guess, not for any motive power reasons), or how many people are making the trip - I guess two people - what matters most here is that the trip should be what he wants to do.
If he has expressed interest in trying out the Caledonian Sleeper, and it's a trip that's within financial constraints, then I say it's worth doing. Plenty could be found to do within the 12 hours or so between arrival and departure on the Inverness sleeper I should think. Some time to explore the city in the morning before lunch then a trip out to, say, Loch Ness in the afternoon before tea and a trip to the pub before departure. Or a trip out to Kyle of Lochalsh, even just a straight out and back trip, it's worth it for the lovely scenery. Off the Fort William sleeper, there's that cable car line that could well be worth checking out, indeed a ride up the stunning Mallaig line is worth doing. Up to Fort William, onto The Jacobite to Mallaig, time for some fresh fish and chips before returning to Fort William for some exploring before returning on the sleeper.
Just some ideas really, but as I say if he's expressed an interest in trying out the sleeper then why not I say. It's an experience that's worth doing at least once in your lifetime, although I must admit to being slightly biased towards sleeper train travel due to it being a great time saving trip as well as raking in the mileage during otherwise dead time!
I'm not saying things like the Orient Express aren't worth doing, crikey I'd love to do such a trip, merely defending the Caledonian Sleeper idea.