Men's fly plaids are almost always seen worn on the left shoulder. There are no "rules" about what shoulder to wear it on, like have been invented for lady's sashes.
(See my article on that here).

Sounds like someone is confusing the men's plaid with the lady's sash.

As the fashion of wearing it on the left comes from the fact that most people are right handed, I wouldn't have any qualms about a left-handed person wearing their plaid on the right shoulder -- but I would suspect that most left-handed men wear the plaid on the left anyway in deference to fashion.