Cycling into tunnels with claustrophobic person
This is how we did it, for all tunnels longer than 150 meters (ranging from 200 meters to 4 km):


before the entrance of the tunnel, the leader slows down or even stop, allowing the follower to get closer
after entering the tunnel with lights on, regardless if there is a cyclable sidewalk or not, every once in a while the leader ring once his bell
if the follower feels ok with the leader's distance/pace, he rings back once. Else, he rings back twice. The follower can also ring twice whenever he feels he should get closer.


The benefit we got:


no need for the leader to turn his head and look back
the follower feels actually in control and not left alone