@Qelix
I don't think she knows. She still seems pretty preoccupied about Human Lento (HL) whereabouts when Monster Lento (ML) left.
One thing I'm concurring with you is that she had no reasonw to keep HL in the dark concerning his worth to the guild, to other adventurers and to herself.
This world has been depicted as violent and unforgiving. Adventurers die often, so often in fact that people don't seem to care anymore. So life is short.
HL was seeing himself as a failure, a useless human with no real redeeming features, and apparently, everybody around him never even tried to say to him that he mattered... or to help him for a change, instead of asking him stuff.
I wonder how many awesome adventurers are alive in this world thanks to him and his tutelage when they were starting. It was described in previous chapters that he actually had a noticeable impact on the rate of newbies' deaths. And despite all that, nobody tried to do anything for him.
Well, except for Lorraine... She actively helped him before his transformation, it seems. She's still a bit of a coward, because it looks like she had feelings for him for a while now, but at least, she helped him.
Also Rina, who actually met him as ML, and still tried to help him as much as possible.
So fuck everybody that knew him before his change (except Lorraine), They can go to hell, the whole lot of ungrateful useless trash that they are.
Those are the kind of people that keep asking for stuff of someone and treat him like garbage, until he breaks and takes his own life, and then they cry, "not understanding" why he would commit such an act.