That's a good scheme for them. One suggestion would be to travel a little earlier in the year, because by the time the Chieftan reaches the northern Highlands (incl the impressive Grampian mountains and the red stone Culloden Viaduct glowing in a setting sun) and then Inverness itself at 8pm, it will be dark. The Chieftan's morning departure time just before 8am won't be quite so dark in Sept., but still shady. Could they do it before mid August?
I agree that First Class is the most tolerable way of spending 8 hours (each way) on any train. Do the 2 who don't have Senior Railcards qualify these cards? They'll reduce the costs, even of Advance tickets by 34%.
Advances are currently available up to 17th July, and the quota of Kings Cross to Inverness Advances tend not to be snapped up straight away; e.g. you can still buy 1st Advance singles with Railcards for July at £48 (Outward) and £40 (Return) per person. Not bad for 1100 miles (return distance)!
Its occassionally possible to save a few pounds by splitting the tickets, however with the limited stops of the Chieftan I don't believe its even worth looking! The only other exploit I know to reduce their costs further would apply if ScotRail re-introduced their Club55 scheme (for Over 55's) - this would cover the Scottish elements of their travel for around £22 per head, return. But I don't know if it will be re-introduced this autumn and as the Southbound Chieftain doesn't stop at Berwick, they's still have to buy EC tickets between London and Edinburgh, which is again, unlikely to save any more cost.
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I see they might be returning on a Sunday, in which case, the Inverness departure is at 09:40 which is in full daylight in Sept. Also, the Sunday Chieftan southbound DOES stop at Peterborough, so it migh be worth spending a few moments to consider splitting the tickets there, esp. if 2 of the travellers don't qualify for a Railcard - but the saving won't be great.
