i have seen this before. a lady come to me and when i looked it was like a small line of blood under the nail that had changed to black. this may of been is she filed your nail to thin and insted of it coming through i think it may be like a blood blistery thing.
hers graduly faded and grew out, but i advised her not to go back to the tec incase it got worse but to keep them manicured, oiled.
**tell the girl what happened so she can learn from this, as everyone does something whilst training. and hopefuly in no time she will be a great tec like many others xx