Get familiarized with the form 2555, 1116 for foreign tax credit on earned income (if applicable) and tax treaties with their country (if applicable). You will almost certainly have to file an FBAR through FinCEN as well, unless she files her own.

Some expat returns are quite straightforward and some can get quite complicated. If she simply has earned income taxed abroad that is excludible through the 2555, that is the easiest case scenario. I guess once you review her tax documents you will be able to determine how complicated her return will be for you!