If she says she's sick, you have to take her word for it.I took him at his word when he said he could translate Russian.
take (one's) word for it
To accept what one says without further verifying or investigating. You're right to be wary, but, in this case, I think we can take John's word for it. He has no reason to lie to us.
take (someone's) word for it
To believe what someone says without investigating further.