@T**
, she had a single entry Non-B visa which was good for one 90 day entry into the country. IF she wanted to exit and re-enter during those 90 days she'd need a re-entry permit.
On a single entry visa buying a re-entry permit doesn't add any more days to the original entry stamp you get.
It allows you to exit/re-enter the country and get stamped in for the same date you got when you first entered on it.