Who deserved it?

He misstated it, but he's not alone.

Kay Kyser once told us:

I got spurs that jingle, jangle, jingle
As I go ridin' merrily along
And they sing, "Oh, ain't you glad you're single"
And that song ain't so very far from wrong

If you break that down and cancel out all the double negatives in the last line, the singer is clearly disagreeing with the spurs and is not glad to be single.

Similarly, I often hear people say things like "I miss not seeing my parents".

The easiest way to understand this is to undo two negatives at a time:

"I can't think of anyone who didn't deserve it more."
becomes:
"I can think of anyone who did deserve it more."
or, more clearly:
"I think everyone else deserved it more."

I don't like multiple negatives, because they are so difficult to parse. One negative per sentence is my maximum.

I'd be far from lying if I neglected to deny that I couldn't help but fail to disagree less.

I agree with telemath. As stated, everyone else deserved it more. He was the person of all who deserved it least. One could easily infer that he didn't deserve it at all.

Still, I agree with Ron's observation that it is misstated because it is so hard to decode. A version that seems clear to me with the fewest changes is: "I can't think of anyone who deserved it less."

But, to be fair to your friend, when the universe is so topsy-turvy as to promote the least deserving, some jumbled word choices are sure to result.

If I may be so bold as to suggest some congratulatory words for your friend to offer his colleague:
"When I consider the others who might have been promoted, your name certainly stands out."
"Since your promotion means you will be leaving our group, I naturally have mixed feelings."