Hypothetically, your approach shot to a par 5 is about 4-5 feet away for birdie. You notice a bunch of spike marks between your ball and the hole caused by someone twisting their soft spiked shoe. It looks like you are just going to have to putt your ball through the obstacle course when fate intervenes.

One of your competitor's lag putts ends up almost on your line but slightly closer to the hole. The competitor marks his ball, looks intently at all the spike marks and then proceeds to smash them all flat.

He has greatly improved your potential putt.

What do you do?

Sounds like if a penalty is going to be assessed, it's against your opponent and not you.

I would thank him....

You kindly thank him and inform him he just got two penalty strokes

What happens if the competitor makes his putt and then taps down the spike marks? I had a situation like this in my league last week. There were some spike marks near the cup where it looked like someone drug their shoe on the green. I putted out and then tapped the marks down as I left the hole. However, my opponent still had to putt and the spike marks may have been in his line. Do I receive a penalty because I improved his line? At the time I questioned myself if it was okay to fix it before he putted but I assumed if I waited until after he putted I was just fixing it for the next guy. I know I shouldn't mess with someone's line but the marks were kind of bad and it just felt like the right thing to do. What is the right call?

You should have waited and was subject to a 2-stroke penalty. Your fellow groupmember should have been given a 2 strokes penalty too for allowing you to do what you did (according to 13-2).

The proper action was to wait until everyone in your group had finished putting and than repair the spike marks.

Decision 1-2/0.7 says:
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...The provisions of Rule 1-2 do not prevent a player from taking acts that conform with the Etiquette Section, so long as the player does so for the sole purpose of caring for the course and without intentionally influencing the movement of a ball, or the physical conditions affecting play, of a player in the player's group or match. ...
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