Can I deduct a gift made to a friend's 529?

Gifts are not tax deductible.  If it was over $14,000 you would need to file a gift tax return in most cases.  If under $14k, then no reporting is needed.

Even though it is a contribution to a 529 college savings plan?

Correct.  For federal taxes, 529 contributions are not deductible, even as a gift.  But the earnings grow tax free for you.   Certain states will allow the deduction, if both you and the 529 plan are in the same state.