It is possible for her to be ovulating while breastfeeding. However, it is different for every woman. Apparently, how soon a woman’s period return is dependent on various factors, such as nursing style, introduction of solid food, bottle feeding etc. You can take a look at this article for more information: http://www.newkidscenter.com/Period-While-Breastfeeding.html According to another article, it is stated that "Changes (in nursing frequency) that are more abrupt tend to bring fertility back faster (e.g., cutting out one nursing session abruptly, rather than gradually decreasing nursing time at that session). This could be the reason why many mothers experience the return of fertility when their child sleeps through the night or starts solid foods. [taken from http://kellymom.com/ages/older-infant/fertility/ -- under "Do I need to wean to get pregnant?" of article] Hope this helps!
She can be ovulating even when she is breastfeeding.