Technically it was Tyrion's plan. He's the one who came up with the whole thing.
All Jon did was volunteer to go with them.
Tyrion, the grand strategist who lost two battles in a row, admitted to Jaime that he never anticipated that he would abandon Casterly Rock, and who formulated what has got to be the single dumbest plan ever put together on this show. But yet after all of that, had the galls to talk about seeing shit from your enemy's perspective so that you could see what their moves were and counter it.
Buddy has been doing lots of this,
this season, and nothing to show for it. Him telling Dany not to save them was just, agh. A few episodes ago he told Jon that he believed him. If Jon doesn't come back with a zombie to convince Cersei to agree to a truce, so that Dany could send her armies north, they're all going to die anyways.