Exact definition of single nucleotide polymorphism (SNP)
In genetics, a SNP (Single Nucleotide Polymporphism) is the substitution of a nucleotide with a different one.
A SNP can have phenotypic effects or not. 

A mutation, in genetics, is any kind of inherited change in DNA. Mutation types are: SNPs, indels, structural variants, translocations and so on.

Sometimes researchers (especially in the field of medical genetics) call "mutation" (meaning "pathogenic mutation") a SNP which is responsible of a disease (especially in case of a Mendelian disease). Since mutations causing severe Mendelian diseases are usually rare, several people use the arbitrary threshold of 1% to discriminate between a "SNP" and a "mutation".

So, probably your professor is stating that SNPs cannot cause a Mendelian disease because he uses the last definition I gave. However, while this definition may be useful for medical genetics purposes (i.e. I also agree that a very common variant is unlikely to be causing a rare Mendelian disease), it is arbitrary, if not wrong.