So her duties, mission, work does not matter?
Her past, beliefs, fears either?

I think what she did is perfectly normal given the circumstances. Not dumb at all.
Even if she probably knows she doesn't stand a chance and doesn't know Hibino is also out of stamina.

Everything about the situation is utterly confusing too. Trying to use a bit of caution even if it comes with luck - ie that hybrid not deciding to kill everyone after all-, is her last line of defence. A very weak one, true.

Also, it is possible to read Hibino's moves as a way to save himself only, she still doesn't know wether or not he becomes an immediate threat.

And she can't be too trusting as Hibino has been around with this secret for so long and they didn't see it coming.

I'm ok with you not liking the scene, but it's not like it's not normal and at least she didn't shoot immediately.

Anyways this is hair splitting, a very minor detail compared to accept Hibino is an hybrid and probably cooperating and keeping him in the brigade.