Tenants replaced a thermostat and caused damage. Now what?

[Jessica LaCroix Theis]

I'm not sure how to proceed here and hoping to get some opinions on how to proceed. My tenants decided they wanted to updgrade the thermostat so they could control it remotely and tried to replace it without ever notifying me - until they damaged the wires so badly that now an electrician needs to fix it. He sends me this info in email as he's left on a lengthy vacation with the tone as though I need to have it fixed for him upon his return. I don't see how I'm obligated financially or otherwise to remedy this for him. Isn't it breaking the lease to alter the property like this? I need clarity on this. TIA