Is the fact that someone warned an employee of a store about a spill important in proving a slip and fall case resulting from an accident caused by the spill?

Yes, the fact that an employee of the property owner (or possessor) was given notice of the dangerous condition is very helpful in establishing that the owner knew of the dangerous condition, and was negligent in failing to remedy it.
James R. Keller
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Partner at Keller & Keller