This issue was fixed in the last update.
Did the host check their multiplayer settings, in the Game section of the options? When starting a new game, you specify single or multiplayer, so appropriate settings can be used; when loading a save, the settings in the options are used. The host can check / change that in the options before or after loading a save, or after loading a save, bring up the in-game menu (Start button) and select the Connection Menu.
Try starting a new game and see if you can connect. Don't move up the beach, though, or it will trigger an auto save which could overwrite a save from your current game (the number of quick and auto saves kept can be adjusted in the Game section of the options).
Try connecting in an old save, then if that works, load the latest save.
If the settings are correct, and there were no system or local network changes, it is possible your ISP(s) may be having issues (or throttling some traffic) during peak hours. Try connecting at at different times of the day, if possible.
Try swapping who is hosting. If it works in a new game, the problem would likely be on either or both ends of the connection.
Have either of you tried connecting with anyone else?