I was playing Tekkit with my buddies, and my friend died after I already typed in stop in the console. Will the items still be there when the server reboots?
If the world has been saved your items should still be there, so using the stop command is safe since the server saves the world before stopping.
I still recommend everyone to manually save the world using the save command.
If the item is in a loaded chunk, it will disappear after five minutes. Chunks are loaded around a player Minecraft unloads all chunks when the game stops, and the items will be saved (including the amount of time they have until they despawn). Even if your friend died after you typed in stop, the game will kick all players before saving, so your items should be safe. If your spawn or any logged-on players are near the items, the items will start to despawn.
When you next log on, try to get to the items within 5 minutes, before they despawn, however if you are far away enough, the items will not begin to despawn until the items are loaded.