Whenever working with gas, the tank should be plumbed with Tygon (or other gas proof line), and zipties, fuel barbs or fuel line clamps used to secure the line.
The supplied clear tubing is only for glow installations and will certainly fail if used in gasoline installations.
Tygon (and most other gas approved lines), while gas proof, does stiffen and slightly loosens over a short period of time. It is common practice to do these things.
Many take it to the next step and solder Dubro fuel barbs to the tubes:
I don't see this as a "tank defect" but rather a lack of experience on the builder's part.
Did your friend use Tygon for the fuel line? If so then he MUST have been aware that silicone is not gas proof and that the clunk line should have been plumbed with Tygon as well.
I'm sorry for your buddy's loss, it sucks to lose any plane, and especially because of an error like this. But I reckon this is the first time someone has lost a QQ 85" Yak due to not plumbing the tank properly for gasoline.