OK, here's a little background first: I recently purchased a 172 with a co-owner that is a guy I have known for several years. He is a newly minted pilot with around 100 hours with most of his training in a Cherokee, but transitioned to a 172 near the end of his training. Myself, I have had my ticket for 13 years and have around 400 hours (Sad, I know). Prior to us purchasing the A/C, I took him up for a flight in it to check it out (an aircraft owned by a friend of mine, and of which I had flown several times before) and although we flew with myself as PIC, he did fly some from the right seat to check out that the plane was satisfactory to him as well. All was good and we made the purchase. Since then, in the past 4 months of owning the aircraft, I have not flown with him as PIC...until this past weekend... The situation: We decided to take a flight together to a nearby Fly-in. It was decided I would fly the leg there, he would fly the return. The flight down was beautiful and non-eventful and we both enjoyed the day at the fly-in. But as I climbed in for my first ride in the right seat of our airplane, a few things not to my liking ensued. First, I noticed he missed a step or two in the pre-takeoff checklist. Non-critical items, but none-the-less, no step should be missed. Diligence is of the utmost importance in my opinion. He then proceeded to taxi without ever touching the mixture, leaving it full rich. Again, not critical, but our airplane loves to foul plugs unless leaned aggressively while taxiing. Something we have discussed together before. He called the wrong run runway designation several times when announcing his intentions over Unicom. Then upon takeoff, he proceeded to climb with a very high AOA and what looked like about 50 KIAS from the right seat, in quite turbulent air along with a high DA. The stall horn blipped for a second at one point. I was ready to take the controls from him at any moment and remained on very high alert. I know I definitely should have said something right then and there but I didn't. Even after the momentary stall horn, he continued to climb at a very low airspeed. Once we got to altitude and leveled off, he still hadn't touched that mixture control, which was still firm against the firewall. At that point I asked him how he was taught to lean and his reply was "We never really covered that." Really??? I then asked him if I could show him and he was receptive of my directions. We then proceeded to discuss a few things he had been taught. He told me his instructor always made him fly full speed in the pattern and stay at altitude until on short final where he was to pull the power and dive for the runway. Again, I was speechless. So much for a stable approach eh? I did voice my disagreement on that. As we approached our home airport, he called out his intentions for the favored runway, then lined up for the midfield TAXIWAY (NOT parallel to any of our runways) and called 5 mi final straight in! I immediately asked him if he truly planned on landing on the taxiway and after a brief discussion he changed course to line up for the runway, but then called our own tail number wrong. I asked him if he uses LGUMPS, he said no. Once on final, he basically did as he said he was shown and chopped power and dove for the runway. He never used carb heat. He did round-out and make a decent touchdown, but in the middle of the left half of the runway, proceeding to comment how he always lands way left of centerline. (My mind was screaming "that should never be acceptable! Fly the freakin airplane, don't let it fly you!") He then left the flaps full down the entire taxi in and used the incorrect control inputs while taxiing in the wind to our hangar. By this point I was furious inside at how such a fresh ticket holder could be so sloppy, but I sat silently. I didn't even know what to say, but was thankful just to be on the ground alive and our airplane not bent up. We put the airplane to bed and went our separate ways. But every since then I cant quit thinking about every detail of that flight, especially the climb-out/stall horn part and how that could have killed us both. I can see the NTSB report in my head. I've kicked myself over and over for not saying anything to almost every mistake I noticed. Don't give me wrong, I in no way claim to be the worlds greatest pilot and am always learning, and I would hope when flying with a fellow pilot that they would mention my mistakes and I would be open and receptive of that. Now I don't know what to do. I know I need to talk to him about it, but want to approach him the right way, as to not just beat him up or sound like I'm nit-picking. After all, I'm not a CFI or examiner, so don't want to come across like that. But, most of all, I don't want him going out and killing himself and/or a passenger and know I never said anything. I also don't want him wrecking our airplane. He has told me how he thinks I'm such a great pilot and far better than him, so maybe he'll take heed to my input. Any thoughts or input???