The situation is as follows- a friend of mine, a teacher working for a private school in Georgia, is pregnant and will be allowed the standard FMLA maternity leave (12 wks without pay), though the school she works at has under 50 employees. However, for the time that she is gone, she has to have all the lesson plans done for the sub, and do all midterm grades, and all report cards. During this time, she is NOT being paid. Is it legal that they expect her to still do that work and not pay anything? This just seems very wrong to me!!

If the employer doesn't have at least 50 employees, her leave is not FMLA leave, regardless of whether the employer calls it that, it's the same length as FMLA would provide, etc.

She's a teacher, it sounds like, which would mean she's more than likely, an exempt employee. If she works a portion of a day, as an exempt employee, she must be paid for the entire day, except if the absence is due to intermittent FMLA leave, which does not apply here (even though the employer is calling it that; it isn't). See here for the definition of "salary basis", which is required for the majority of exempt employees, and the situations under which an exempt employee's salary may be docked.
http://www.dol.gov/dol/allcfr/ESA/Title_29/Part_541/29CFR541.118.htm

As long as they leave her on exempt status, they have to abide by these restrictions.

As it is not FMLA without the 50 employees, then it is legal. Essentially her employer is granting her 12 weeks of leave from her teaching duties but requiring she perform certain tasks.