After some more searching, adding this in the client makes it work:
require('https').globalAgent.options.rejectUnauthorized = false;
/**
* Client
*/
var io = require('socket.io-client');
//var socket = io.connect('http://localhost', {port: 8088});
require('https').globalAgent.options.rejectUnauthorized = false;
var socket = io.connect('https://localhost', {secure: true, port: 8088});
socket.on('connect', function(){
socket.on('event', function(data){});
socket.on('disconnect', function(){});
});