Minecraft server logs - does anybody know what this means?

[13:39:42] [Server thread/WARN]: Skipping Entity with id diamond
[13:39:42] [Server thread/INFO]: [grapesgreg: Unable to summon object]

I found this in my minecraft server logs - I have a suspicion my friend may be cheating - I have no idea what this means, and I'd love to know.

Guest

It seems, according to me and my Minecraft knowledge, your friend is trying to get(summon) diamonds, but was unable to get them. So yes, he is cheating.

Lava