It may also be worth mentioning it to the mum.
Sometimes parent come out with the swear words without realising that the kids are listening. I was horrified when my DD, then only 3, came out with a phrase I know that I use but thought I had not not used in front of her. (It was nothing to bad but I don't think it would have gone down well at nursery)
I explained to her that sometimes people say things that are not really very nice, especially if they are cross, but that she should not repeat them as she would probably get into trouble. I then developed a new phrase "oh bubble gum" that if repeated would not cause as much trouble.