Absolutely gorgeous! I had a week there and my mother several, Harry. Omanis are very, very friendly and love Brits, which is good for a start. The hotels are very civilized and you can certainly get a drink in them, and excellent food. All the streets are wide, safe and clean. There is plenty to do once they find their feet with the expats, and, if they're so inclined, they can find people to take them out into the surrounding countryside to meet super villagers. The women do not wear the dreadedblack full cover abayahs, but very colourful dress and straw hats.
There are wonderful mountainous regions and flat plateaus to thunder around in 4x4's, with loads of wildlife - wild mountain goats, oryx, baboons, great fishing, super sea sports/snorkelling/swimming, and thousands of birds - resident and migratory - to view.
The Sultan's palace is the focal point of the city and as your uncle's building a hospital, I'm sure he will know all the best parts of town to show them the fascinating old-style Arab buildings - most of those are now lost to neighbouring countries in their desire to look like downtown Chicago, so do tell them to be sure to take a good camera. They mustn't miss a chance to take a trip on the Gulf-style fishing boats known as dhows and see if they can spot any turtles and dolphins.
They can wear 'normal' Western clothes and should find the weather very pleasant this time of year. It'll be warm, with even the possibility of some early rains kicking off, but cotton and light jackets will be sufficient. It's a very charming and civilized country.
Wish them a wonderful time from me!