How many diamonds can a Diamond Ore drop in Minecraft

I hate to break it to you, but your friend is correct here. Without any sort of enchantments or mods, diamond ore only drops 1 diamond. There's a high likelihood that you might have simply miscounted the number of ores you mined in single player.

Note that the average number of diamonds with a Fortune enchantment are 1.33, 1.75, and 2.2 for Fortune I, II, and III respectively.

Well... Acording to the wiki :

Diamond ore mined using an iron or diamond pickaxe will drop a single diamond.

Simple answer: Your friend is right, you are wrong.

With no enchantments, a single diamond ore will drop a single diamond, no exceptions.