this IS a triggering topic. as such, please use self care when reading replies/posting your own .

my 2c only: i would take steps to protect the child e.g., contacting the mother as a first step & assessing her willingness to take responsibility to figure this one out. if the mother doesn't seem like she is going to act, i would go talk to the child's teacher.

as for the veracity of whether the situation actually occured, i would remain agnostic until someone actually investigates. just as i think there is a danger in disbelieving the account, i think there is also a danger in believing what was said actually occured until it is investigated further. i personally would be careful not to let myself think about the "whats" e.g., who it was, if that person was old/young, where it happened etc because the story may very well turn out to not be true (and then again it might be true, in which case those details would come out to people who need to know in its own appropriate time).

edit: also emphasise to your child what a great job she did of coming and telling you!! austin-t and i have been talking about this issue lately (encouraging kids not to keep secrets, what to do when they say something) so it'll be interesting to see what other people think.