She would go mad if she stayed.

As there could be no chance of escape for Percy now, as he had agreed to meet his deadly enemy face to face at a given place, and a given hour, she could not be a hindrance to him: and she knew enough subterfuge, enough machinations and disguises by now, to escape Chauvelin's observation, unless...

unless Percy wanted her, and then she would be there.
No! she could not be a hindrance.

She had a passport in her pocket, everything en regle, nobody could harm her, and she could come and go as she pleased.

There were plenty of swift horses in the stables, plenty of devoted servants to do her bidding quickly and discreetly: moreover, at moments like these, conventionalities and the possible conjectures and surmises of others became of infinitesimally small importance.
The household of Blakeney Manor were accustomed to the master's sudden journeys and absences of several days, presumably on some shooting or other sporting expeditions, with no one in attendance on him, save Benyon, his favourite valet.