Your friend is wrong, there is no way that is a cat failing.
Your car has 2 cats, one for cylinder 1, 2, and 3 and a second cat for cylinder 4, 5, and 6.
See you your cylinder 2, 3, and 4 are in both cats and there are 3 other cylinders without problems.

Coil problems are common, but 3 at once is odd...
Did you replace the plugs at 100k?