Understanding what you are doing and what you may not want to do to the leather may help, identification of the leather is first, if he uses a solvent then on some leather it may take the top coat finish from the leather therefore leaving it without any protection and it will re soil quickly and the dye may even peel off.
LTT sell an ink stick I would use that, hair spray is too uncontrollable, petroleum jelly is another but using the correct solution for the job will yield the best results.
Shaun
PS Judy did Barkers Furniture call you ?