My DD is very sensitive, so any time we bring up a new worrisome concept for her we try to frame it in a way that won't lend itself to a new global fear of dogs or fire or whatever. She is 4, so one thing that works really well is to frame things as a simple problem she can solve. That makes her feel more in control. I would have 3 concerns for this conversation-I don't want my DD to fear her dogs or other dogs we meet (but always take an opportunity to reinforce our dog safety lessons), I don't want my DD to be afraid of her friend's wounds, and I don't want my DD to hurt her friend's feelings.
So for this I might say "Do you think our dogs would ever hurt someone or bite? (no) That's right, we have to be really nice to our pets and train them and raise them carefully, so that they will not bite. Doggies who have been hit or been treated meanly a long time sometimes DO bite. That's why we always remember to ask the owner before we approach a strange dog, to make sure it is a friendly dog. A dog who was not friendly bit your friend Sally right on her face. When you see her, you will see some marks where the dog bit her. She might feel bad about those marks on her face. Can you think of ways we can help her feel better when we see her?"