Damages caused by tenant-wear and tear or gross negligence?

[Kayla Marek]

Our tenant from hell will finally be gone at the end of the month. Unfortunately, last night he broke the push lawn mower. Initially he told us the blade "fell off" while he was mowing. Under further review, it appears he somehow sheared off the entire crankshaft. This mower was only one year old when he took possession of it at the beginning of his lease. I got three quotes today and all were far more expensive to repair than the actual cost of the mower itself. The technicians also informed me that this is not consistent with "wear and tear" and is a result of gross negligence in the form of mowing over a large rock or tree stump. All three companies stated it is impossible to cause this type of damage simply with routine mowing. 

We informed the tenant of the above information and that we would be deducting the cost of the replacement from the security deposit at the conclusion of their lease. They denied hitting anything and disagree that they should be held responsible as this was "clearly an accident" (which to me is an acknowledgement of guilt?)

I feel like we have all our bases covered in the event they want to take us to small claims court. Just want to get the Bigger Pockets community opinion as we are still fairly new to this. Any thoughts?