Opponent intentionally kicks my ball off the green

True story.  Yesterday, (on the 7th of Torrey Pines North), I'm about 18 feet above the hole and furthest from the hole of the foursome.  I had marked my ball and there was another ball near mine.  I placed my ball at my mark and putted, my putt went 15 feet past the hole.  As walking towards the ball I said, "wow that's fast".  My opponent who was below the hole kicks my ball off the green while sarcastically saying, "so we're no longer following the rules of golf".

This hole had already become my blow-up hole, this was particularly unnerving.

What's the ruling?

(BTW, he explained that he thought I was taking a practice putt to see how fast the green was.  I've play with him maybe 20 times and have never taken a practice putt on a green while my ball was in play,)

It was stroke play, we were playing for money.

Terms?  Stroke play, lowest score for the round wins.

I'm talking about my opponent intentionally kicking my ball off the green before I had finished the hole.  Certainly that's a breach of the rules and there should be some sort of penalty.  As for me, I made my best effort to replace my ball at the spot it had come to rest before he kicked it and finished the hole.

What is the penalty to my opponent?

Per Rule 18-4, you are not penalized (you need to replace the ball where it was and continue).

I think that per Rule 1-2, he would be penalized 2 strokes (possibly disqualified, although it didn't really benefit him so probably not).

Just for future reference, if it's stroke play then he wasn't your opponent, he was your fellow competitor, or FC in forum shorthand.   Using correct terminology makes answering rules questions a lot less confusing.

If he had been your opponent in match play then he would have incurred a one stroke penalty.  In stroke play he incurs no penalty unless he makes a habit of it.  In such a case the committee might be justified in disqualifying him for a serious breach of etiquette.

I'm curious - why isn't this a breach of Rule 1-2?

1-2 . Exerting Influence On Movement Of Ball Or Altering Physical Conditions

A player must not (i) take an action with the intent to influence the movement of a ball in play or (ii) alter physical conditions with the intent of affecting the playing of a hole.

Exceptions:

1. An action expressly permitted or expressly prohibited by another Rule is subject to that other Rule , not Rule 1-2 .

2. An action taken for the sole purpose of caring for the course is not a breach of Rule 1-2 .

Even though the bet is only between me and him, he's not an opponent but a fellow competitor....

Not straight foward. My memory is just not good enough to remember all these nuances.

It's not that tough, match play is you against someone else, i.e. your opponent.  Stroke play is both of you competing against the course, thus fellow competitor.

Not quite.  Match play is scored hole by hole.  Once the play of the hole is over it's as if those strokes never happened.  All that matters is the state of the match between you and your opponent.  Match play can also be contested between two sets of partners, or between a 3 singles each playing matches against the other two (which can get a bit confusing - not recommended for beginning match play players).  The one constant is that hole by hole scoring is always used to determine the state of the competition.

It is very possible for a stroke competition to be played just between two players or as a fourball between two pairs of partners counting only the best ball for each hole score, or as a foursome (alternate shot) played by totaling strokes for the one ball being played by each pair of partners.

Stroke play was originally used only when tournament fields were too large and unwieldy for match play, but it is no longer restricted to large fields, and it isn't always just played against the course.

What action did he take that would influence the movement of the ball in play or alter physical conditions?  In the scenario described, he did neither.  The ball can be replaced and the situation would be as if the fellow-competitor never touched it.

Some examples of a breech of 1-2 would be waving a towel at a ball as it's moving, to try to change its direction, or stepping on a ball to sink it slightly into the green, ruining your fellow-competitor's putt.

Why would you voluntarily play golf (let alone for money) with someone who competes with this mindset without a purse of at least several thousands and referees to keep him honest?

Agreed.  This is one of the most bizarre stories I've ever heard.  Who the hell takes practice putts before they've finished the hole?

I've played with guys before who, if they didn't have time to putt on the practice green before the round, will mark their ball on the first green, and then hit a couple lag type putts (usually away from the hole) to get a feel for the green speed.

.... and they no doubt are honest enough to add the four penalty strokes they have incurred to their score.

They rarely keep a real score.