You're playing with someone who puts a ball marker down in front of the ball on the line of putt. Is that legal to do? Yes, under Rule 14 , you can mark a ball anywhere on the golf course so long ...

The procedures come in Rule 9.7, which talks specifically about a ball-marker being moved, but with some cross-referencing back to Rules 9.4 and 9.5 which relate to a golf ball being moved.