Your friend is wrong on that one. The honorific "Ms." is the general term used when addressing a woman.
In the US it's used to refer to a woman regardless of marital status. Since you would use "Miss" to refer to a (younger) lady who is not married, and "Mrs" to address a lady who is married (regardless of age), "Ms" can be useful as a catch all term to use if you don't wish to cause offence by guessing whether the lady is a "Miss" or a "Mrs".
In the UK, Ms used to be reserved for a woman who had been (but was no longer) married, although now the US and UK meanings seem to have merged.
Also, it's pronounced "Miz" (at least in the UK), so is pronounced slightly differently to "Miss", which is worth knowing.