This alone is an indication that the site is a scam.  Legit exchanges often ask of KYC if some suspicious activity is seen on an account.  They willn ever ask to pay a fine because they can just easily deduct the amount on the fund held on that frozen account and they just need the consent of your uncle to do so.  Instead they asked for extra money.

If your uncle had deposited an amount on that exchange, then I am sorry to say that most probably that fund is already lost and your uncle got scammed by that exchange.  Tell him to never pay a fine because it will only add more losses from your uncle's side.