It's possible but we would have to see the code that is being used on the web page. If it is open source and we can review that the keys are generated locally on the users browser then it's unlikely. However if the user receives the keys by other means (like email) then it is possible for the third party to keep a copy of the keys.
RE: Justin Sun Thwarted - A White Knight saves the Steem stolen by HF0.23 by sending to Bittrex.