Let me ask a more basic question. According to your local electrical code who is qualified to change an outlet? If, according to code, you are not "qualified," then you may be opening yourself for liability by doing the favor of changing the outlet. A proper connection for a three prong outlet requires 3 wires, hot, neutral, and ground. This may be a case where you would be better off not doing the electrical and just tell the customer they will need to get an electrician in to redo the outlet. Once the outlet is redone, then you can come back and finish the installation.