She did it to keep herself safe. Remember how it says she was just a "disposable" person?
Yeah, i know but she should not sacrifice other people happines for her savety especially since the boy is still innocent ╮(￣▽￣)╭
She's not exactly sacrificing him. Telling the empress about his training won't kill him. Not like she told him where he secretly goes... But if she is useless, she will lose her head.
Yeah he wont die, but the empress will definitly do sth about that information, right? And she might restrict the boy movement and the boy's future knight might not be the merchanaries and he might be killed in the war coz he has different knight (●￣(ｴ)￣●).
There are a lot of information from the novel that she can use, but she choose the mc boy secret. ┗( T﹏T )┛