While Alxmrphi is correct, "Are they better off there?" could be interpreted as "Are they richer there?" even though it doesn't necessarily mean that.
Perhaps it would be safer to use: "Is it better for them there?" or "Is life better for them there?".
Unless, of course, it is a matter of money.
I am not sure what country they emigrated to but we do use 'better off' in a wider variety than just dollars.
We regularly read of children put in a foster home where they will be better off. Or we hear of people who have moved to a new part of the country but they are no better off. It could mean health, wealth, or any of a variety of things based on the context.
Sometimes the context is unclear and one will ask why they are better off (or not). It is not always assumed to be money in the bank.