Ooh! I shall be in Cambodia then, insh'allah, but, hmm. If they can manage it, I would absolutely recommend trying to get to Siam Niramit, as it is visually stunning and gives a rather fabulous overview of Thai history, culture & beliefs in a very accessible and enormously entertaining way. (Plus, the price of the ticket includes an acceptable all-you-can-eat buffet.) But that will depend on what time they're there - it's an evening thing, obviously.
An alternative location for the evening would be Suan Lum Night Bazaar, in Lumpini, just round the corner from where I live, which includes a nice foodcourt and cocktail bar and is wall-to-wall shopping fabulousness. Possibly not their cup of tea, though? But I love the living hell out of it.
Night time is fabulous for taking cocktails at the Vertigo Moon Bar, which is a spectacular open-air bar/restaurant on the roof of the vast Banyan Tree Hotel. One can sit sipping mojitos and looking out across the glittering cityscape in style - always fun.
Needless to say, they can watch women sticking cigarettes, razors, bottles, pingpong balls and God knows what up their ladygardens in Patpong, which is also v. close to where I live. Patpong is also good for buying pirate DVDs & suchlike, but they absolutely will be approached by touts trying to lure them into various bars and clubs and suchlike for sleaze.
Daytime touristy sites very well worth seeing - The Grand Palace and Wat Po springs to mind as the most obvious Not-To-Be-Missed location. (Also rife with scam artists, of course, who will try to convince them that the palace is closed for prayers, or for a special festival, and that what they really want to do is get into their tuk-tuk and go visit a load of jewellery shops that have very special discounts just for them. These people should be glowered at and stomped past, for they lie like lying dogs that lie. But once you're inside, everything's rosy, and the official tour guides are lovely.)
The Blue Elephant is a terrific place for truly elegant, delicious and utterly luxurious luncheon, and all you have to do is get on the Sky Train and get off at Surasak Station. Probably the easiest place one could possibly find.
Actually, this is a pretty good suggestion for a one-day Bangkokfest.