Legal Question in Credit and Debt Law in Pennsylvania

You have answered your own question. If the other party resides in the United Kingdom, then you will have to find a lawyer (could be called a solicitor if in England) to assist you.

You cannot sue the person in the United States unless jurisdiction could be asserted against him/her. And even if you could sue, there is a somewhat cumbersome process to go through for service of process (you have to go through the Hague Conventions on this). Further, the person would have to have some assets in the United States that you can attach. For these reasons, it may be cheaper in the long run just to get a solicitor. I have no idea as to what they charge for their services or whether the Facebook chats are sufficient or whether the claim would be within the time limits.