you can fix it in two ways:
1. reroll and derive the pubkey when creating a new secret key if it gets a 3 prefix compressed 33 byte pubkey
2. subtract the curve order G from the secret key to get the inverse which has the opposite sign and a 2 prefix
this cannot be fixed by users or by clients, it breaks either one direction or both directions
one user has 3 key, their messages can't be decrypted
both users have 3 keys neither user can decrypt each others messages