Assuming there was no simple typing/scrivener’s error, then making a deal with the neighbor seems like the only solution.....maybe a prescriptive easement.

Some websites sell "certification" for support animals not issued by a doctor, BUT there are now websites with actual doctors selling service animal prescriptions, and legally we must accept them.

They just informed me that their son has been diagnosed with autism and the doctor gave them a letter/prescription advising that he have a "comfort dog". 

They get a prescription for the service animal and then never go back to the doctor.4.

If they can prove they have used your land, had their fence there, etc. with no objections, then they may claim prescriptive easement. 

The agreement can be as simple as to say the 'property owner A' and property owner 'B' hereby acknowledge the proximity of their respective driveways and affirm that neither property has any easement right/necessity over the other.The concern here from a lender/title perspective is the ability to claim a prescriptive easement right.

You likely have a prescriptive easement for it since its been am open and notorious use for 30 years.

I have a feeling that it will be a prescription dated after the incident. 

I will request a doctor's note and/or prescription (and registration of ESA if available).

Worst case scenario is that they will gain a prescriptive easement to your property because you knew and neglected to do anything about it.