I feel a bit daft asking this

[Swango1980]

Yes, you can mark someone else's ball. Think of the scenario where they are from another group and are looking for the ball, but no idea where it is. They may never come and claim their ball on your green. It would be a sad state of affairs that you couldn't mark it and it was directly in your line.

 

[clubchamp98]

Yes, you can mark someone else's ball. Think of the scenario where they are from another group and are looking for the ball, but no idea where it is. They may never come and claim their ball on your green. It would be a sad state of affairs that you couldn't mark it and it was directly in your line.

Yes I thought that you could be there all day!
The old fashioned styme ( should bring in a form of matchplay that these are allowed ,just for fun)

 

[duncan mackie]

If B had played from on the green, the same penalty would have applied. NB It doesn't say anything about the other ball being a ball in play

If the player’s ball in motion hits another ball at rest on the putting green and both balls were on the putting green before the stroke, the player gets the general penalty (two penalty strokes).

See definition of "in play" (thought this had a had a long airing?)

However, whilst the other player may have significantly exceeded his strokes for the hole, or been DQ from the hole for playing a ball from a wrong place (SB) with his 5th stroke, i dont understand why it wouldn't be considered a ball in play in the context of this rule anyway 🤔