Ink on leather is not always as straight forward as people would like to think it is. The combination of types of finish with the number of different inks makes it impossible to produce one product that will work on everything, anyone telling you anything different is just blagging.
Ink is a dye so it has effectively re-dyed the leather.
If it is on a pigment suite the first port of call is an ink stick, the quicker you get to the problem the more likely you are to get it out. Beware of which one you buy some don't work on anything and others are so strong they will strip the finish off.
If the ink stick does not work then it becomes a technical problem as a solvent based product needs to be used with great care to remove the ink. There are different strengths and how strong you have to go then dictates the amount of finish or pigment you will need to replace. The final stage if you cannot completely remove the ink would be to recolour over the top.
If the ink is on an aniline then there really is no chance of removing it as it will have soaked into the leather and you cannot get it out.
If you want to give me a ring on 01423 881027 then I will be happy to help further