I would definitely get your girl fixed (either abort the litter if you are ok with that, or wait until after she has the litter). I would also call the neighbor and tell them that they may want to get their boy fixed so this doesn't happen to anyone else.

Crossing my fingers for no kittens, but in our experience it is very easy for them to get pregnant. My fiance's mother's cat, Cali, got pregnant before she was even a year old, and had a litter of 4. She was fixed not very long after the litter.