Hey everyone,
I too was experiencing A TON of problems getting co op to work with my friend. We eventually figured it out. I hope you can follow my steps and see if this resolves any issues for you, as well.
First- Click on Start
-> Computer
-> Network (on bottom left)
-> Under Network Infrastructure right click the picture of your router
-> Click Properties, followed by Settings
-> Add. Description of service is Dead Space 3, Your IP for your router ex. 192.168.<>.<>, both ports should be 3659, select UDP, Click OK.
Now, the second step is to portforward. The best way to do this is go to google.com and type something like "Portforward for Dead Space 3 PC <insert your router name and model here" and submit.
Your results should be something like this:
http://portforward.com/english/routers/port_forwarding/Linksys/WRT54G/Dead_Space_3.htm
Or you can simply go to http://portforward.com and navigate to your router type and select the game there.
Follow the very easy instructions with setting up a Static IP, and then opening the ports that it prompts you to in your router settings.
I know, it seems like alot, but it only takes about 10 minutes or less to set it up, just follow the pictures, it's very easy and you'll be playing co-op in no time.
Feel free to add me on Origin or Steam and message me for any help, I'll be glad to attempt to walk you through it.
My user name is Repticon on Origin and Bronald Schwarzebrotha on Steam.
Good luck !