ok well theres a thread floating about where a guy had this issue and got screwed
read the small print on your paper work for the real deal
Read the Magnuson Moss Act for the real deal. It is the law in this country.
I haven't seen this thread you talk of on here so I don't know the whole story to accurately comment on it but in a general way, assuming there was engine damage and an aftermarket oil filter was used, the following applies...
I assume when you say "the guy got screwed" you mean warranty coverage was denied on an engine failure of some type. If so...
The car mfg could legally deny the repair if the filter itself failed and caused the damage. In that case the car mfg is not required to repair the damage. If that was the case the guy should have gone after the oil filter mfg as they would be liable for the repair.
If the filter itself didn't fail however then the car mfg was legally bound to honor the warranty as long as the guy was using the correct filter and did his OC's on time. If that was the case the guy didn't stand up for himself or was not educated on the law and let the dealer/car mfg get away with something. In this instance the guy screwed himself.
Aftermarket filters( or other parts/fluids )do not void warranty just by their use. The filter itself has to fail and directly cause the failure. That is the law all car mfg's in this country are required to follow. Unless the car mfg provides the oil filters( or whatever part/fluid )free they can not require you use OEM brand to preserve warranty.
To deny a repair because of aftermarket parts the car mfg must be able to show/prove the damage was a direct result of said aftermarket part/fluid. THAT is the real deal.
NOTE - this applies to the factory warranty the car mfg provides with your vehicle. The same rules do not apply to aftermarket warranties that can require more of you.