In addition to the usual pilot currency stuff (FR, IR) he just needs a 2C medical and LDG currency (day or night) for pax.
100 hour inspection does NOT apply since the plane is owned by the pax for his own use. This is a very simple scenario because the customer is providing the plane and hiring a pilot to fly it.
If the pilot, or someone else, provides the plane, then you are holding out, running a 135 op, or a 121 airline, depending on a variety of things. This gets complicated fast.