This is absolutely 100% false.  Non mining nodes still reject invalid blocks, and would thus only download and validate valid blocks.  The 51%er would thus just do the equivilant of create a hard fork which only he can use, while the smaller sub-network would create valid blocks that user clients would accept.  So all of the non-mining lay users still wouldn't even see the invalid blocks, they'd just notice that confirmation times have slowed.