Assuming that there is a lightning network transaction between two people and they want to make evidence to be available to another person as witness, you can send the txid of bitcoin transaction to the third person which the person can use to trace the transaction on blockchain.
It can neither be used as evidence. How can the witness verify that the merchant indeed requested his coins to be sent at the address provided by the client?
Assuming that there is a lightning network transaction between two people and they want to make evidence to be available to another person as witness
What's the use case here? Is it to be able to prove fraud, for instance when a seller says you didn't pay, while you say you did?
Yes
I can only think of one way, but very few merchants do it: use PGP. The merchant signs a message including the address he wants you to send your coins in. Later, you send them. He cannot deny that's his, as you have a signed, solid evidence.
No, you can't use this in LN. Only the peers (the two LN nodes) can have this information for themselves, and cannot prove it to a third-party.
