OpenKeychain does this, provided both users have it of course.
Re:I'm giving up on PGP
by NoGravitas on Wednesday December 07, 2016 @10:26AM
(#13123162)
To expand on this, OpenKeychain wants you to use the camera on your phone to scan a QR code on your friend's phone.
Re:I'm giving up on PGP
by hackermailman on Tuesday December 06, 2016 @11:19PM
(#13120429)
https://www.cylab.cmu.edu/safeslinger/ does simple handshake exchange and works w/iOS/Android but unsure if still maintained
Basically, if you can swap contacts via NFC then the pgp keys should go along with it.
It may have some theoretical weaknesses such as the exchange being MITMable if the users don't verify something on their screens, but I think having many more edges in the graph would make up for it since you might already have an expectation for that key through friends-of-friends paths.