Maybe use
makeup remover? Leather is skin, after all.
But seriously, are we talking about coated or uncoated leather? Either way, I'd gently wipe off the lipstick or whatever it is with a soft cloth. Then use a moist cloth with a tiny bit of saddle soap and try removing the stain in a circular motion. If an oil stain remains after the area has dried, apply an absorbent powder (ie rice flour) for a couple of days, then vacuum up the powder.