Make sure you ALWAYS get a consent form signed, especially one where they have to initial next to every possible complication. Takes longer for them to sign, but it will cover your butt. Then ask them if they have any questions.
Perfs happen. We've all perf'd teeth and the saying is true, that you haven't done enough endo until you've perf'd a couple of teeth.
When discussing with the patient, just say "I'm having trouble finding the canals due to calcification, funky anatomy, etc so I really NEED to refer you to a specialist." Do not touch the handpiece anymore. If you've found a few canals, insturment them with a 10 file. Place CaOH, cotton pellet, and temp material. Take the tooth out of occlusion. Take radiograph. Give them referral and pain meds. I wouldn't charge them for anything at this visit. Call endodontist and tell them about perf and let them finish the endo.
Document everything. If the tooth is restorable, proceed with crown/build-up. If it's not restorable, offer to pay for implant directly to a surgeon or perio, then restore in the office at no cost. Patients are generally understanding that stuff happens, they just want you to make it right. Plus, if a patient tries to sue you after they've taken your services for free AND you paid for the implant, they're likely to get laughed out a courtroom. Just don't give your patient MONEY.