Yes A Heter Iska could be drawn up for this loan.
Let's call your "Jewish fellow" JF, and the pawnshop owner PO.
If the money was not yet given to the borrower, The heter Iska would set the transaction to be an investment by PO to become a partner in the profit generating business of JF. They money which is subsequently paid by JF to PO is not interest but dividends on his investment. The "Rate set for payment is a pre-agreed amount which the investor PO agrees to except and not obligate JF to prove with witnesses or swear that there was no more profits generated. A small Part of the money given by PO to JF is given as fee for JF generating profits from PO's money.
If the money was already given as Aa loan to JF, then besides the Heter Iska, a Kinyan Sudar (an article given from PO to JF) and in return JF gives PO a partnership in his profit generating venture as stated above.
In order for the Heter Iska to work, JF must either be using the $50,000 for a business venture, which can reasonably generate profit eqal to the amount which he will be giving PO as dividends. or he must stipulate in the Heter Iska that PO is becoming a partner in any other profit generating-asset which JF might have, if he has another asset which might generate that amount of profit.
Both parties should understand the workings of the Heter Iska transaction, and agree to the transaction being as stated above, and not think (as, unfortunately many people do), that A Heter Iska is just some religious document which has to be signed in order to permit lending money with interest.