disinterested, uninterested

Disinterested or disinterested means neutral or impartial; having no stake or interest in the outcome of a contest or dispute, in a practical way--
They called on a disinterested party to arbitrate the dispute.
He was often a disinterested third party to the arguments of his two friends.

Uninterested or uninterested means not interested, or having no desire to watch or participate, in an emotional way--
They left the game in the middle, since they were uninterested in the outcome.
He tried to catch her attention, but she was uninterested.

Now that you know that you can say--
"He was present as a disinterested observer, and so was uninterested in the outcome of the contest."