There are several alternatives to Seroquel, like a good lithium supplement and the Iodine protocol, for a start.
Maca; no problem as I see it, but in my experience it wont do the big changes. Did the herb for a few months but couldnt stand the taste of it. No side effect I could notice. Not much energy either; Iodine is far better here IMO.
Compared to the meds she take; the mismash of supplement can hardly cause much harm.
I notice that several of the symptoms taking Seroquel seems to be a deficiency of magnesium; I would start here long before Maca. http://bipolar.about.com/cs/sfx/a/sfx_seroquel.htm