After all the things XLH have done and they've been through, I didn't expect she still chose to do that (yeah, I know she is threatened by this antidote thing) but she should have told her the truth if she really value their friendship at this point.
When anyone defies DFQS regarding his future bride, one threatening key word : DIE + death glare and they all shut up, that's still the vilain lord we all know lol๐๐๐. Habits don't change for some people