I never understood why blackmail is considered a good way to force someone to party up with the blackmailer in manga (and some other media I've consumed). Surely the simple act of getting blackmailed would keep the MC on edge against her? That'd lead to a lot of distrust, something you don't want in a party where the party member's lives fall on each other working honestly and wholeheartedly as a team.
This should be easily recognizable to her, unless she's an airhead or something. So basically through the act of attempting to blackmail him, she just completely disincentivized either of them from actually forming the party. Goofy stuff.