First I thought "what a stupid woman", but ACTUALLY, she made the right call.
If that saved the mc from dying/losing his ability to fight back, then that was the only choice she could've made.
If mc's dead, then she's also, but if she takes heavy damage/loses a limb in exchange for the mc to have another chance at it, then she also might live through it all. It was clear anyway that she's pretty useless against that boss herself.
Might as well be a tank for a sec and suck some damage.