The official website you'll need is:
Bahrain Electronic Visa Service
Your best bet would be a tourist visa - once she's in the country, she can get it extended before it expires. They don't seem too bothered about western expats extending their stay once their in the country.
Eventually there will come a point when they wont allow you to extend it any further and all you'll need to do is the all too popular 'visa run' - fly to Dubai, then back to Bahrain and you'll be issued with a new tourist visa.
A bit long winded I know.