RE: DLE60 Twin
Help can only be as good as the information provided. It wasn't until the last post before you said anything about the carb being fully cleaned. Thats a fairly large piece of info don't you think.
If you have ruled out contaminated fuel system components, jugs, lines, gas, etc. the only thing left is the plane itself. Maybe the carb is sucking in something from the cowl. Some debris or film on the inside of the cowl. Was it painted by the owner, a baffle added that introduced a foreign matter of some sort. I certainly have never heard of this.
At this point I would like to see pictures of the complete installation.
But it sounds like you are done. So I hope your friend gets it figured out.