[Sorry misread this and have changed my response slightly top the following..!]
Hmmm -- this could be a serious issue for her -- because SHE sounds like she has a problem dog here.
I'm not quite sure what actually happened–what do you mean by saying one dog submitted, and another dog didn't? If the other dog was friendly when it initially came up and took no notice of your dog that was friendlier (I wouldn't really call this submissive, it's just a better socialised dog that is good at reading signals and is friendly with other dogs and will give appeasement signals
) but you had another dog on a lead that went for the larger dog, a well behaved larger dog might do a lot of barking, and look fierce back, without ever intending to actually bite (even snapping is not actually biting, it can just be a kind of annoyance response–the issue is whether there was intention to actually make body contact and inflict damage–which Codi definitely was trying to do). What did Codi do that wasn't submitting and that initiated a response?
What is worrying is that from the description -- which I may not be understanding correctly -- it seems Codi is capable both of trying to bite somebody else's dog and actually to bite a human in the midst of a fit of aggression, even if by accident. Has this ever happened before? Most dogs send signals right away of friendliness in any meeting with another dog. The worry is that Codi did not though I am not clear what he actually did.
Just even on the basis of a single incidence, it's potentially a fairly serious issue to work with a trainer on to try to lower the reactivity level, because that seems a very severe level of fear aggression (a dog being off lead & coming up to meet other dogs isn't generally going to be seen as a problem especially if it wasn't initiating any kind of aggressive encounter but responding to aggressive signals). In other words the owner of a large dog that came over to visit that suddenly had their dog attacked or confronted, prompting a response, would not be the one who was in the wrong–it would be you(in this case; your friend, normally) for having a very reactive dog that is unmuzzled out in a public place, even if on a lead, because the indications would be the dog cannot be managed/controlled on a lead alone and will bite with intent.
Another complication though is interpretation - it can be quite hard for an owner not really familiar with body signals dogs send (which most of us are not) to tell the difference between a dog that is just kind of snapping in self-defense and one that was actually going to try to seriously hurt another dog, But it is still the case that you had the dog that -- if I am understanding right -- began the confrontation. If the other dog was not under voice control and wandering around, then the situation at best would be that you were both equally at fault, but it would not be the other owner who is primarily at fault here.
Just by contrast–I also have a mildly fear aggressive dog, Lily, who can be quite rude when she meets another dog initially–she will growl and bark right in their face and sometimes lunge at them, but she would never actually bite–she is just stupidly rude and other dogs can read the difference & ignore it. I have no doubt that if they could read that she was intending to bite, there would have been problems many times over. I've worked to desensitize her to some degree, but I know to keep her on the lead and well away from other dogs and if I see another dog coming I just lifuallr. On the other hand, if she gets her 2 to 3 min. of yapping at the other dogs, she then is perfectly fine and can run around and play in a group with them and there are no issues. I have had her behavior assessed by trainers and it isn't considered serious aggression but just bravado and can be managed in an easier way (and she has gone to daycare and also goes to home boarding, and these little bursts of mild aggression settle down almost immediately and have never resulted in a problem with other dogs). So there are different levels of fear aggression and a reaction that someone might see as an intention to bite, actually often isn't, it's just some back-and-forth posing.
Once you have ever had a dog that has literally gone for another dog, especially one that, in the over excitement of trying to attack a dog, has actually bitten a human, you do (or your friend does) have a problem. Most dog wardens would not hesitate to order the destruction of such a dog if there were a formal complaint either by the owner of a dog that was attacked, or a person who was bitten, even if accidentally in a melee.
Maybe I have read this wrong (and you can give other details about the actual encounter) but even taking the best possible view here the black dog would not be the only one at fault, it would likely be seen as an equal level of responsibility. Hence I don't think I would rush to file a complaint.
But what a horrible and unexpected experience all around for you!
