How could it be proven other than by a typical signed agreement? Testimony, emails, text messages, videos, recordings, bank statements, forum postings, etc.
I don't know how the plaintiff in your case will prove it, obviously, but the point is that it's possible. In fact, unless you're willing to perjure yourself, it sounds like the existence of a contract is not even going to be an issue. The primary dispute may be over the amount owed, and who knows what the plaintiff has up her sleeve to prove that.