she should probably just deal with that shit and get her community service over with. aside from that... as long as she's not working for a place that deals with a lot of money or require her to go into peoples homes, they usually don't do background checks.
And no, if you get hired through one agency, they won't keep checking your background as it costs them more money. The only reason they'd check again is if shit came up missing or she was acting questionably.
Chances are they won't,it depends just apply way and see if they hire you. A friend of mine worked for a temp place for awhile and then out of now where the place found his recent arrest. A temp job or some retail place don't bother with a back round check most of the time.
What type of charge would help? As some misdemeanors are as weak as financial responsibility law (no insurance) but some could be theft charges. Companies hiring do 3 possible types of background checks.
1 is a in depth as far 10-15 years.
2 a very weak and non detailed background check. Most likely major drug crimes, felony theft, etc
3 Is no background check or say they will into scaring people with a hidden history away. As I'm in a sales/marketing career (legitimately). I've found that established companies hiring non entry level employees don't even drug test. A resume alone pretty much says what kind of past you've had.