Ouch that is sad someone didn't treat the leather or interior well. I'm eternally impressed with those magic sponges, but had them leave a white residue on the dark lawn furniture. We use Lexol products
How to Clean Leather with Lexol Leather Cleaner and Conditioner purchased locally, read their site info over to start to form an idea of what you want to do.
But to clean up neglected surfaces I know we have a member who is very knowable and works for Adams on this forum. I know many people who use the Maguires NXT series products and are very happy.
If it were mine I'd have a container of clean water and soft cloth ready, a magic sponge and a Lexol cleaner ready. I'd try the sponge in one small area, then wipe with the cloth/clean water, then follow it up with the Lexol leather cleaner last their finish product. I have seen a vanilla steering wheel on another classic yellow that is so trashed I'm not sure it can be restored. I put a white padding on ours then a "Wheel Skin" in black. I really love the vanilla wheel but having it covered/protected in black has been equally nice.