The California mom who fought off a deranged attacker while changing her baby in the bathroom of Macy’s Herald Square was going off “a mom’s instinct,” a family friend told The Post.

“I’m not surprised she did that,” the friend, Justine Silva, said.

“She works out. She’s pretty fit,” Silva said of the brave mom, who was stabbed in the back, shoulder and arm in the Thursday incident.

Kerri Aherne, a California mom, was attacked by a deranged attacker while changing her baby in the bathroom of Macy’s Herald Square. Obtained by NY Post

The mom and her family were supposed to leave NYC the day after the attack but are now stuck in the Big Apple with no clear date of return, Silva added.

“It was a holiday thing,” Silva, a high school girls soccer coach who knows the family through her boy

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