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They need to be certain to hire a LICENSED contractor who is familiar with DOT specifications for sidewalk repair work or else it can end up costing them more in the end by having to do it over. They can find and/or check one out by calling 311 or going directly to www.nyc.gov/dca.
If the cause of the damaged sidewalk is due to a tree's overgrown roots they should call 311 for an evaluation for the City's Forestry Dept to make the repairs no cost to you.
Any additional answers to their questions on sidewalks may be found at
http://www.nyc.gov/html/dot/html/faqs/sidewalkfaqs.shtml