Based on all of the responses and Hulk's info, here is what I would do:
tell the new customer "I know you're a friend of (insert current customer's name here), and I want to help you out. But i have another client who has me booked through the weekend."
Then, offer him a quick wash & wax, or even offer to use a glaze to help get his car ready for whatever he needs it for on Saturday. Further, offer to book him next week for a full polishing. But sweeten the deal for him a bit since has to wait. For example, you could offer him a free interior cleaning as part of the service.
By doing this you:
1) Somewhat satisfy the new customer's needs by saturday
2) Completely satisfy your existing customer's needs without compromise.
3) Get a jump start on next week's bookings. You've effectively turned this new customer's one emergency into two appointments!!
This way you don't sacrifice anything, but the customer may still feel like he's getting more than he's paying for. In my example he's getting some expedited service (wash & wax/glaze), and he's getting a small perk (free interior cleaning). At the same time, the time constraint and the risk to your existing customer base is minimal.
tell the new customer "I know you're a friend of (insert current customer's name here), and I want to help you out. But i have another client who has me booked through the weekend."
Then, offer him a quick wash & wax, or even offer to use a glaze to help get his car ready for whatever he needs it for on Saturday. Further, offer to book him next week for a full polishing. But sweeten the deal for him a bit since has to wait. For example, you could offer him a free interior cleaning as part of the service.
By doing this you:
1) Somewhat satisfy the new customer's needs by saturday
2) Completely satisfy your existing customer's needs without compromise.
3) Get a jump start on next week's bookings. You've effectively turned this new customer's one emergency into two appointments!!
This way you don't sacrifice anything, but the customer may still feel like he's getting more than he's paying for. In my example he's getting some expedited service (wash & wax/glaze), and he's getting a small perk (free interior cleaning). At the same time, the time constraint and the risk to your existing customer base is minimal.