Your case and question don't portray sincerity and it's incomplete at the same time. Is the wallet custodial or noncustodial? Do you still have crypto assets in it?
Based on these, if you don't have crypto assets in it, why waste your time, go for another wallet for privacy and security reasons. But if you have crypto assets in it and it's custodial, contact your wallet company with your original details for a reclaim. However, if it's noncustodial, signing a known message with your private key will be the most viable way out.
Let us make it maximum easy.
We both know seed-phrase. How can i prove that the wallet is mine???
Here is the maximum easy answer. If you both have the seed phrase then the wallet is both of yours.
Explanation being that ownership of an address comes down to who knows the private key and I guess in legal situations who used the key
If you both have the private keys then you can both use them. I dont think theres been court cases yet where people dispute the ownership of an address and both have the keys. The only court cases Im familiar with are the ones where someone is trying to claim ownership without having the keys.