No, only the owner of the car can recover damages. In addition, your fiancé cannot appoint you to represent him because only an attorney can represent another individual in court. Your fiancé would be the sole proper party Plaintiff and he would need to appear in court. Your witnesses would need to appear as well as notarized statements are hearsay and inadmissible.
If you had collision, it is much easier to make a claim against your own insurance company.
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You need to be the owner of the car in order to have standing to sue. Notarized statements are not enough as this is basically hearsay. Those statements are not subject to cross-examination. You need the witnesses to be in court.