In this situation, there is little difference in the information given. The guy will be here on Friday, and that's all you need to know. However, there is a big difference between the prepositions of time 'by' and 'on'. For example, let's say you have an IT problem at work, and your maintenance department tells you this: 1. The problem will be fixed by Friday. 2. The problem will be fixed on Friday. The first sentence means that they are going to fix it during the week, maybe Wednesday or Thursday, and that the system will be already fixed and good to use when you come to work on Friday. The second means that the maintenance people will be busy fixing the system on Friday- probably meaning that it'll be out of use that day. So, as you can see, the difference can be very important indeed.

They mean pretty much the same. The difference is petty.

They are both correct and mean slightly different things: (1) "He will be here by Friday" focuses on his arrival. He will arrive no later than Friday. He might arrive earlier. (2) "He will be here on Friday" does not focus on when he will arrive, but rather says that on Friday he will be here. He might have arrived on Friday or he might have arrived earlier. But it's not important.