I also agree with Andy but with the added thought that has also been mentioned - if the neighbour turned her mares out in season in the proximity of a stallion and nature took it's course then she as landowner also has responsibility for the risks taken and her part in maintaining appropriate boundaries, be they the fencing, a secondary gap maker fence (electrified for instance) to keep her mares a safe distance or more sensible by far, putting them somewhere else when blatantly in season. A stallion can not be blamed for what his hormones do if she is aggravating him either intentionally to create trouble and get rid of your tenant, or through bl**dy minded ignorance. It is always the people the law should look at, not the animals doing as nature intended. I would suggest you look at her part in it and mention the possibility of a counter claim for the damage to fences and trouble with your tenant that affects business income. She might be using it as a means of getting rid of the nasty stallion - some folk are like that, and if so I wouldn't give her the satisfaction.
Honestly it's all about sueing the pants of everyone for everything these days - folk used to have more sense than to taunt a stallion, and if there was an "accident" then half the time it wasn't accidental but means to get a free service, but nobody sued for it, just dealt with it or kept the foals and sold them. Just saying it isn't always the stallion's fault, nor his owners.
Also if you willingly entered a grazing agreement for him then yes you accepted liability - it's one reason a lot of liveries closed down when the law started to pursue livery owners for welfare costs for abandoned/miskept horses, not to mention they still don't get to keep/sell the abandoned ones whose negligent owners left lots of unpaid bills..
Stallions aren't nutters by and large, but a reasonable precaution you presumably took or would leave yourself way open for, is an electric enhanced fence at the boundary of grazing in all directions but particularly doubled up if possible where others are close by. For my stallion the fence between him and my other stock has elec wires offset both sides and every alternate post is extra long/deep to prevent casual knock downs. The march fence only had normal post and stock net/wire with one elec wire, but now my neighbour is keeping pet ponies (mares at that, the stupid man) across that single fence it's driving my lad demented so I have to move him in May and he can't go back til October at least. I'm about to add a secondary boundary with a double neck length gap and plant hedging in it to reduce risks for future years as that has been my foal field for 12 years due to being level, overlooked by my office window and the best size of what I have for the stocking level it gets.
I am convinced my neighbour is waiting for him to serve the two wee pony mares and get free foals as he previously asked me the stud fee and I declined on the grounds those mares are a welsh A type and a shetland/welsh type and to me they're dangerously small to be served by my stocky Highland, not to mention the guy wouldn't pay me a bean, I know his rep..