1.Kill your best friend several times.
If your friend is better than you at PVP the best method might be spawn killing. You can build a not-so-simple machine so that your friend will be pushed into a deep hole when he respawns but the downside is that your friend have to die to respawn so you might need some assistance from your other evil friends.
This is the easiest way to ruin your friendship. Just go into their base and steal it. But if they keep their valuables in their inventory you have to combine it with the first method for better results.
3.Killing their pet (if they have one)
This task might not be very easy because they might protect their pets even if it meant dying. So to make it easier we should wait outside their base because if they are in survival they will need food. But if they have chests full of steak of something like that we have to use the most cruel method. By building a simple TNT cannon we can destroy their base with ease and kill their pet. But if they have extreme security measures like a 5 layer obsidian wall the only possible way of killing their pet is by mining into their base using a pickaxe while they are not home.
This task might be even harder because it has a very low chance that your friend will trust you. You have to drop a dripstone into the end portal and convince your friends to go in. And the higher you drop the drip stone from the more damage it will do.
This task will not be possible unless your friend trust you enough. Step one build a fake secret base that has a fake entrance and only you know the real entrance. And make it so that the water at the bottom of the will be switch to lapis every time someone jump in. Step two make your friend use the fake entrance. (Sorry about not putting a tutorial I can't find one and idk how to make it)
If you are a good builder make a replica of their house in the sky and show them a video of burning it. They will immediately come home and see a untouched house and will be even madder.
This one might be cruel so if you are soft hearted just skip this one it's no use for you. Step one build a glass container. Step two kidnap their pets. Step three capture a warden. Step four kidnap your friend. Step five make him set his respawn point or you will kill his/hers pets. Step six let the warden into the room. Step seven watch him suffer. Step eight use admin rank and put him in adventure mode. Step nine kill his/hers pets anyways.