I'm very sorry to hear about your pup . Small dogs often do trigger the prey drive (to chase and harm smaller animals) in larger dogs, especially if they aren't around smaller dogs much. I wouldn't doubt if that's what happened in this case, eventhough Rotties usually are friendly towards all animals.

If the owner agrees to pay for damages and accept responsibility for what his dog did, then no, I wouldn't invlove animal control. If he isn't willing to though, I'd definitely involve animal control. I wouldn't make him pay for the entire bill, but most would. I think it's entirely up to you.

I have a small dog (along with 3 larger ones - a Rottie, a Dobe, and a retriever mix) and have only ever had two dogs come after her while out for a walk. Both weren't used to small dogs, and she triggered their prey drive. I wouldn't expect it to happen often.