Technically, she can stop studying as soon as USCIS receives the properly filed I-485 application.
However, stopping studies (without graduating) before the interview can create a bad impression and set you up for a hostile interview. The shorter she has studied, the more likely it would be a problem to stop, because quitting too soon raises suspicion of student visa fraud (i.e. it looks like the student visa was just a pretense to get into the US and marry and immigrate).
So if she has studied for less than a year, I would strongly advise continuing her studies at least until the interview. But if she's completed a year, that should be enough to show that she's a bona fide student, so it would be less of a problem to quit.