IQ is good. AF is noisey, slow, imprecise, fidgety. MF is very hard to do accurately. If your shooting style allows you time to get the focus right then it is superb value for money. If you need to shoot quickly and accurately (think wedding), especially wide open with shallow DOF in poor light (think wedding), then it is the wrong lens to choose.
Build is ropey and the lens may well disassemble itself if dropped/knocked. You really don't need your lens to be falling apart while shooting a wedding! It's not like you can go back the following week and try again with a replacement lens. If you have the funds then the 50/1.4 is the better bet but if you get the 50/1.8 and want to trade up you probably won't lose much on the sale of the 50/1.8.
FWIW I have had the 50/1.8 for over two years and I only have two pictures that were taken with this lens (neither one very good) from my library of ~6,000 keepers. I rarely need faster than f/2.8 but when I do, the AF on the 50/1.8 does not inspire me with confidence that it will deliver reliably sharp images, especially for moving subjects (think B&G walking down the aisle) where focus tracking is necessary. I now have the 50/1.4 and 85/1.8 for those more demanding low light situations.
I think that for shooting the wedding preps, for free, and with the cooperation of your friend, the 50/1.8 will do you just fine. For professional wedding work, or under higher pressure for speed/reliability, I'd pass on the 1.8 and go straight to the 1.4.