Upvote:3
As far as I know, what you're heard is correct: you can only stay in the Schengen area for 90 days of any given 180-day period. It will thus not be possible for your mother to return before the 180-day period expires, and note that this period starts from the day she physically entered Europe, not when the visa validity started.
That said, she should be able to apply for her new visa before the 180-day period ends, it's only concerned with actually physically entering the country.
See also: How does the Schengen 90/180 rule work?