It's hard to gauge the condition without looking at it first, and the color of the interior leather may dictate the degree of carefulness to be used.
Hopefully they haven't tried to remove it and you can pinch the leather between your index finger and thumb and try to roll off as much paint as you can. Sometimes this technique will allow the paint to come off in bigger chunks
Last year, I worked on a Saab 900 that the owner has spilled nail poish on the passenger seat
I used the technique described above, and got some Nail Polish Remover Acetone Free available at Walgreens to clean up the rest.
This worked real well for me.
Again, depends on the color of the leather, you may want to first try on an inconspicuous area. The key is not to use too much down force (hard rubbing) as to not smudge onto clean areas. Just work it slowly and assess the progress
If possible, use a white towel. That way you can see check for colorfastness. (Easier to see that way...)