This is one of the most common questions I am asked, your friend is correct a non-immigrant visa multiple entry issued by a Thai consulate outside of Thailand means that for the duration of the one year visa you must cross a border every 90 days, the most common way is to go on a Cambodian visa run which will automatically give you a further 90 days, this must not be confused a one year extension visa which was awarded to you by a Thai Immigration Officer within Thailand i.e. retirement visa, business visa you can report to an Immigration office every 90 days.