Don't go near short stay, or the pick up area that is inside short stay. You will be fleeced for money, and risk having your car towed if you leave it unattended.
Go to the Mid Stay, and either park up and get the bus up to meet them, or tell them to get the bus down to Mid Stay.
If you are in and out of the Mid Stay inside an hour, it is free. If you go over the hour it is a pound up to the end of the second hour.
However do not stay inside for more than two hours, or you will pay the daily price of £18. Just drive out before the time is up, and drive in again.
If you are in mobile phone contact you can tell them which zone you are parked in to meet up. You also then have the option of killing time at the motorway services which are at the nearest motorway junction (J8), until they ring to say they have landed.
If you are not in mobile phone contact, then you should both agree to aim for the 'Free Set Down Area' in the Mid Stay. bus stop 13.