First, make sure that you have a replacement tile.
You need to ensure that you remove any physical connection between the damaged tile and those surrounding it, because when you prise it off, you do not want to disturb the others. This means removing all the grout, or creating a channel in the grout, down to the surface, so that the tile is an island, so to speak. How you create the channel is down to what tools you have available.
Then, depending on the the strength of the adhesive, you can use a wide chisel, lying across a wooden batten on an adjacent tile and lever the damaged tile off, or simply tap the chisel underneath the tile and ease it off the wall, that way. There are various ways, but it is imperative that the adjacent tiles do not move. Any movement in a tile means that it is no longer set and waterproof and must be removed, cleaned and re-adhered.
Best of luck. Oh, don't let your friend near your shower, again. In fact, judging by the workmanship, can you be sure that the tiling is as good as it should be for a wet area? Did he seal the wall and use decent grout?