In the Klei folder located in the Documents folder you may see some names like: Cluster. Inside this folder contains the world generated by each slot, for example: Cluster_1 (Slot 1). The world is in the Master and Caves folder. You and your friend can do some tests to solve this problem, as it is something unusual, but in doubt, improvise.
Test 1 - After it attempts to spawn the world, the button changes to Resume World, then tell it to go to the World Cluster Slot folder, and check that the Master / Master and Caves folders are filled with the world files. If the files are there, the world is being prevented from proceeding. Here are some of the alleged reasons: 1 - Permission to host, 2 - Generated world corrupted.
To solve the host problem, it is necessary to see some details: 1 - Administrator permission for the user, 2 - Check if the computer has enough requirement to be a host, because a host consumes much more memory than a user. (I believe that firewall can not be, because as it is to host, the firewall would not block, only if it was inbound connections)
To solve the corrupted world problem you can follow these steps: 1 - Generate the world in some slot and then send the world to your friend (Master / Caves), and then tell your friend to delete the Cluster folders.
Here are the folders you should transfer to your friend. Example:
In the same folder : Klei\DoNotStarveTogether\Cluster_1 create a file named cluster.ini
In this file paste the following code:
Create a file in the same folder named: cluster_token.txt
The token can be generated within the game by clicking the Profile button and clicking the button responsible for generating the token, after that paste the token in this file, and tell it to start the world.
Test 2 - If in case his problem is not per host, he can try to host the world with dedicated server. As there is already a post on this, I will be leaving the link below. I recommend generating a world with Caves, because it gives to enjoy much more thing.
My English is not very good, so I counted on the help of google translate because I'm Brazilian, but I hope you understand and help you.