First check leather type. Is it pigment, aniline, wax pull-up, two tone leather with poor quality finish etc.?
Has customer done any obvious damage with fairy liquid, as this product should not be used to clean leather.
At this stage it is unlikely that consumer based ink lifters will make any difference, but they are worth a try as long as it is a pigment and always pre test.
No doubt someone will tell you about hairspray...........it might work but you will need to be prepared to fix the damage if it doesn't.
Your next line of attack would now be solvent removers followed by touch up or re-finishing.
We have many notes on our technical website about the problems of ink removal so feel free to register for this site at
www.LTTsolutions.netSorry there is no simple answer as ink removal can be a complicated process if not dealt with very quickly, ink is a dye which soaks into the leather and is difficult to remove.
Regards
Judy