This is a confusing situation because you were essentially acting as a "Property Manager" for the grandfather, which is an agent who helps the landlord and can act on behalf of the landlord. However, you put your name on the lease, which can also be argued to make you the landlord yourself.
There is no statutory law that speaks to this situation, so it would be on a case by case matter.
The tenant can argue that you were the landlords, and you deceived the tenant by claiming that you had a right to lease. Even if you did not deceive him, by putting on your name on the lease, you became the landlord and since the lease is ending early, the liability is yours and they can sue you for the difference in a comparable lease for the remainder of the original lease's time.
Someone in your situation can argue that you were only acting as an agent for the landlord (see above), and that your name on the lease is a clear mistake and one that the tenant knew of, and that the tenant always knew that the grandfather was the actual landlord. As such, it is the grandfather that is really breaching the lease, and he should be pursued, and not you.
Then, the Court decides what is fair here, after hearing all parties. I will be honest - if your names are ***** ***** the lease as landlord, then the Court is likely to view someone in your position as the landlord as the overwhelming presumption is that whatever was signed was agreed upon and understood by both parties. However in the end, it all depends on the situation and what all parties claim. If the tenant admits in Court that they always knew the grandfather was the actual landlord, then the Court may shift the liability to the grandfather.
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