That's a tough question, as you can't really change the structure of the natural nail plate. The best suggestion I have read came from Doug Schoon and he states that the correct moisture level on the nail plate is vital to healthy nails. CUTICLE OIL (IMO) is the best product to apply to keep the nails flexible. Too many nail strengtheners on the market make nails so hard that they become very brittle and snap off.
INM has a product that will cross-link with the natural nail plate in approx 10 days to help aid in strength and flexibility - but for my money - its cuticle oil all the way!
I believe that CND has a product that can help as well - PM Gigi or Samuel Sweet and they can help you out there!
If the client is having real trouble growing any length I would suggest a natural nail overlay to protect the nail and give it strength and prevent breakage.