J.M.
My sister was a biter. And generally biting occurs when they are frustrated but can't communicate so that people understand, so they bite to get someone's attention. You, of course don't want to necessarily wait until she bites again, but when she does remove her immediately. we had to isolate my sister for a time. Of course, we kept re-iderating that she can't because she bit so-and-so. Maybe they have a separate area that can be gated off.
you could try teaching her baby signs. to see if that helps her communicate better.
If she's an only child she may be struggling with sharing at daycare whether it's space or toys.
Because at some point the other little boy may clock her back. And while you don't want to promote that, it may be him knocking her down for her to understand what hurt means.
If you watch her behavior right up to the infraction you might be able to discern something that she does that might be a key to either re-directing her or thwarting a confrontation. However, it may be very difficult depending on how many kids the center has compared to staffing. If she's truly only doing it at daycare, then you need the staff to give you more detailed info other than "she bit joey because he was in her way." Insist on it. It will probably mean they have to keep a dedicated chaperone with her for a period to try and determine what's causing it.
For instance, is it when their going to snack and she's not wanting to wait her turn. I do think many daycares anymore get very lazy about really determining the cause & effect of problems they just want to pass them off as the parent's issue, but how are you to know unless you take off work.