It happens a lot, so don't worry, it maybe that she has grown tired and just needs a break and will come back to Islam, or that she is searching for something different.
Different religions offer different things to different people. For Islam it can be very hard on women, and maybe she wants something with a bit more freedom.
As for me I've been 3 different religions so far, and I know one OAP who has been just about every religion in the book.
As for the technical side of it, it depends if she was Baptised or the equivalent in the religion. If so then some other religions wont accept her into their midst because of it, but it entirely depends on the religion. The Baptism is the first step then confirmation, I think as long as she hasn't done the later then she is OK. Some religions except all, and don't worry about it at all.
I wouldn't worry and just give her the support she needs, as it will be a very unsettling time for her, as there must have been something to change her mind connected to Islam.