He is actually making a tutorial for Solus (not just one mechanic, but the entire game). You should check it out:
https://wiki.unrealengine.com/Solus_C%2B%2B_Tutorials
What erWilly means by "When the player overlap the trigger use a interface to send the info to the player. " is that you should make a campfire blueprint that has some sort of box/sphere component that when the player stands in it (Overlap Event) it will trigger a function or event that adds to the Hypothermia variable. You could use a Blueprint Interface, Cast Call or custom event to communicate to the player BP.
See these links:
Blueprint Interface
Custom Events
User Tutorials
I hope helps