In the short term, you either need to quit helping your friend or gut it out. I am actually in a similar situation at work where I am doing a lot of work in tearing down a greenhouse but I have to use poor body mechanics simply because they are heavy awkward objects. My elbows and wrists hurt a bit because I have to pick up large pieces of metal/concrete and carry it at an odd angle for my wrists. So your complaints do not fall on deaf ears :o).
In the long term, Jae spelt out a lot of what you can do to help. Scott Sonnon's Intuflow and Pavel Tsatsouline's Super Joints are great programs too. Super Joints is a book, I believe.
You have three types of "muscles" (I am just going to use that term to keep things simple even though it's not 100% accurate). You have real muscles, major movers like your lat or glutes or triceps. These muscles contract or relax and cause muscle movement. Tendons are a connective tissue that connects these muscles to bones and helps in movement. These can get sore but they more often get tight. Your Achilles tendon is a common example and for a lot of newly barefoot runners, it will tighten up hard because of the increased workload. Lastly, you have ligaments which connect muscles to muscle and act as stabilizers. They don't contract or move weight but hold a joint stable to allow the major movers to do their work. Your knee and shoulder (rotator cuff) are two major examples. When you over-extend a ligament, it's not much of an issue. BUT when you over-extend it AND expect it to stablize the joint, THAT'S when you begin to see straining. You suffer from a sprain when the ligament is compromised and it reacts quickly to protect itself. For most people, the tinsel strength of those ligaments is more than the actual power in the major mover muscles so they will rely on the ligaments to provide support in unnatural positions (i.e. shrugging your shoulders up, putting extra stress on your rotator cuff).
In the short long term (I love oxymorons), there are a few things you can be more aware of while doing the heavy work and even going forward. In terms of wrist position, your "neutral" position is when your thumbs point forward. Having your palms facing forward or behind you is actually a flexed position for your wrists, the least stress is when our palms are facing your side. I mention this because when you hold your hand in that position, you can see the natural position of the wrist, upright and not leaning in or out (if your know fitness jargon, the wrist is not in flexion or extension). This is the strongest position for your wrist because it's in a neutral position and the stabilizer ligaments can stabilize at minimal length. In order to protect your shoulder, imagine you are holding a stick in both hands and you snap it in half, putting your wrists into a neutral position. When you feel your shoulder suck down into the socket, that's all "packing your shoulder" and you are allowing the stabilizers (your rotator cuff) to do their job without being compromised. If you have the mobility to use your shoulders in this position, you will actually feel a lot stronger. To avoid hyperextending your elbows, try to keep a slight bend in them when holding something heavy. We had a lot of sore elbows during my EMT training because people kept their arms completely straight during traction spliting.
Lifting heavy weights, even doing partial reps and isometrics, can help strengthen your stabilizers long term but that won't help much today. Keep on keepin on and try to keep good form and eventually the crap work will be done.
