If you have a Napa Auto Parts or Carquest close by, pick up 2 bottles of Duragloss Leather Shampoo. If this is not an option, the Mothers Leather Wash foaming leather cleaner (2 bottles) can be found at Auto Zone and some Walmart stores. If you make the trip to Walmart, pick up a horsehair boot brush to agitate the surface with the cleaner. Then simply wipe dry with a microfiber towel. You can use an APC but the pH is high so plan to neutralize the surface after.
When you turn the vehicle over to the owner, please educate them on aftercare.
1) Use a soft brush such as a horsehair brush to remove dirt/debris
2) Vacuum the seams
3) If a cleaner is needed, use a dedicated leather cleaner and gently clean the surface
4) Hydrate weekly with a water dampened towel and allow to air dry
As stated above, conditioner on those seats is a waste of time and money. It will only leave a film on top of the coating to hold dirt/grime, which ends up grinding into the coating.