Falling through the map is usually a sign of connection issues and I am going to assume both of you are on PC.
Make sure their internet is stable enough (use Ethernet if you have to). Wi-Fi can be unreliable.
If they or you are using a VPN, disable it.
You both should add your game folders and save folders to the exclusion list of the Antivirus program using. Also log into your router and make sure that any built-in security apps are not interfering with any connections.
Whoever is hosting should port forward.
If all the above fails (hopefully not!), they can try the Network Workaround option in-game.