ok so what you are asking is that you want to know that if you leave a patient you have already been treating and let them guide the ambulance in? is that what you are asking?
Once you have assumed care (having been called by security dispatch to respond) and then you leave that patient on their own to guide the ambulance crew in, I think brings up a whole plethora of issues.
1. you are making sure that you are practicing inside your scope right?
2. You have assumed care for the patient as an EMT/security officer correct - then you have established a patient care relationship and you should not leave.
3. You should be directing others on the scene (if any) to go meet the ambulance, leaving to go guide the EMS in, is leaving the patient. That could construe that this is abandonment.
4. What would happen if the patient crashes while you are guiding in EMS?
My advice, once you have established patient contact, you should not leave the patient. EMS are smart cookies, they should be able to find you just fine.