Gatwick Airport

Your friend can't enter the secure part of the terminal without a boarding pass. You can exit the terminal if you wish but that means you will pass through immigration twice (exit & reentry), and security heading back to your gate.

How long is your layover? He will not be able to go past the security area. If you leave that area, you will have to be re-checked. If you have a very long layover- say several hours, you might could leave the airport for a short visit.

It's been awhile since I've been through Gatwick, but my recollection is that there's no place to sit in the terminal after you exit customs, which you'll have to do to see your friend. The Hilton is attached to the South Terminal so you could go hang out there; I've stayed there a few times, but awhile ago and I can't recall whether the lobby is ok for that but it has a couple of bars. If time allows, that would be a good option.

There is not much seating at Gatwick, but what there is is upstairs on a place like a mezzanin. If I remember right, that is also where you can get coffee, etc.

If you take the free and fast connection to the North Terminal there is a very nice Sofitel with several bars and plenty of seating in the lobby, all under the high atrium. Very nice place to chat, I assure you.