It's pretty common for a mare to do this. It's usually an imbalance in hormones. I have one retired broodmare who makes milk every year when the other mares have babies on the ground. We let her be until all of the others have foaled, as she will gladly be a nurse mare if one should reject their foal. We've only had one maiden mare over the years that did reject her baby and it was a blessing that good ol' Buttons was more than willing to take over. We started the foal on colostrum we had in the freezer and then she took over.
Don't know about mare magic. Putting her on regumate would take care of it, but it is pretty costly. If the vet isn't concerned I wouldn't worry about it too much unless her udder gets hard and hot, then you would want to have her checked to be sure she wasn't developing mastitis.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.