I cant recommend a nail strengthener, but i would recommed buying OPI nail oil, and massaging it into your hole nail- including the base of your nail/cuticle area at least twice a week.
Its a two pronged attack of the problem, as both the oil and the massaging itself stimulate growth, and healthy nail beds.
If you dont want to spend lots of money, you can do the same thing massaging your nails, but use good old fashioned olive oil! - massage, leave it on for half an hour, then rinse off.