I'm hosting, my partner is the one experiencing the crash.
I'll have our entire group waiting in our Lower Cityy camp. When my partner joins, we "leave" the camp to enter Lower City, and he crashes instantly.
If he tries to join the game while the whole party is in Lower City already, it either crashes or doesn't load.
Have tried a lot of things, and before patch 3 just turning on slow HDD worked well enough.
As I was writing this, it seems sending my save over and him hosting instead of me fixed it, BUT only after numerous attempts.