[greiskul]

You can prove it, you have the signed transaction from him that can't be included in the blockchain because their inputs were spent. If you know who you were dealing with that did the double spend, you likely would have a very good case against them of fraud.

[pdpi]

There's two potential issues: Can I prove they control the private key that goes with that transaction? Can I prove that that transaction was originally used to settle a specific invoice (or something equivalent)? Sometimes, one of the two will be true and I have a strong case. In others, I have neither and it's my word against theirs.