As a firm initially substitutes debt for equity financing, what happens to the cost of capital and why?

My niece boyfriend borrowed money from me and refuses to pay me back. I did not make him sign anything because I trusted him and my niece was present for the verbal agreement. It is now a year and I have not receuived any form of payment and he keeps promising to repay. He has bought another car but it is in someone.s name. What can I do?