Former Dallas Cowboys linebacker Steve Octavien recently landed a marketing job, got married, brought his newborn daughter home from the hospital and is saving up for the down payment on a house.

But as he gets on with life after six years of professional football, the 29-year-old Mr. Octavien regrets handing over $80,000, including his signing bonus, to a stockbroker named Mary Wong in 2008. They met while he was playing at the University of Nebraska, where he says she sometimes paid his rent, cellphone bills, car insurance...