R3 grabs foul ball

[soundedlikeastrike]

Sounds like you got away with it. But I'd a added to the runner "don't do that".

You say the ball was 6' foul, well what about 6" foul, how about a bunt rolling up the line just 1" into foul ground?

This is a batted ball, until it passes 3rd, "then" it's either fair or foul not an instant before. Would you call "foul ball" on first contact with foul ground, no.
You wait until it passes the bag.

The ball is foul if "it" contacts something or someone while over foul ground.
Not foul when it's "touched" by an offensive player.

The difference when a coach does it is the proximity to the bag, coaches are usually even or beyond the bag/s. Runners are usually several feet down the line. Squelch this action right away.