How are mixed race marriages received, and is my friends experience unusual?
DW and I (white male, Japanese female) have never had any problems at all; however, we have only ever been visiting, so not much experience of dealing with the bureacracy there.
My parents neighbours are a mixed race family - the man is white, his wife is black and the children are therefore mixed race. They have been there for quite a few years, and seem to like it. The wife has told me that once or twice there have been some rather red faces when older, white people from her husband's work have visited. When she opened the door, their first assumption was that she was the housemaid, and asked something like "Is sir in, or madam?".
I have heard of quite a few instances of black people and Indian people, especially men, being rather rudely turned away from nightclubs apparently on account of skin colour. I don't frequent nightclubs in Dubai, so I can't comment on the veracity of that.
It is normal for many countries that actively try to attract expats for one reason or another (e.g. they don't have enough qualified people themselves, and people would not necessarily want to move there without compensation) to make do with someone who would not have been good enough in their country of origin.
I don't agree with the implied assumption here that all expats are living abroad because they couldn't succeed in their own country.