Gerda Theron fears that womanizing Penn will break her daughter's heart, reported Showbizspy.
While Gerda has never minded the pair's friendship, sources say she "begged Charlize not to let it turn romantic."
Gerda has been gently reminding Charlize, 38, that Penn just took a trip with his ex-wife Madonna to Haiti, where the pair reportedly hooked up.
"Charlize refuses to believe it happened. But Gerda worries about her, like any mother would," a source said.