tree damage

What is the name of your state? Indiana
A friend mowed my yard and did weed eating along my property line and the small pine trees the neighbors planted a few feet from this line were damaged. They say these trees will die due to cuts on the trunks from the weed eater and are suing me in smalll claims court for the cost to replace these trees. Am I liable, or the person who actually did the weeding? Is a person responsible for replacing trees before they actually even die? How do I know if they do die, it is from damage, as they have had to replace two of these trees previously? Must one replace them with the same age tree?
Must I pay someone to replace them or can I do it myself?

"Am I liable, or the person who actually did the weeding?"
*** You would be liable to your neighbor, as the 'mower' was your 'agent'. You would then have a claim against the mower.... for his negligence in not protecting the neighbors property. I suggest you add your 'mower' into their lawsuit.

"Is a person responsible for replacing trees before they actually even die?"
*** If the plaintiff can prove the damages, you could be found liable.

"How do I know if they do die, it is from damage, as they have had to replace two of these trees previously?"
*** You can certainly offer that as a part of your defense 'plan'.

"Must one replace them with the same age tree?"
*** Nope. The damaged party can simply accept compensation.

"Must I pay someone to replace them or can I do it myself?"
*** That is entirely up to the damaged party.... and whether they will allow you on THEIR property to replace the trees.