I think that I would stick with the facts, in the order that they have happened, and in as much detail as possible. Explain that as the buns problems have become worse you went for a second opinion else where. Ask why it was not diagnosed and treated in the two appointments you have had with them and state what you would like done about it i.e. refund on their treatment and training course for vet in question, or something to that gist.
ETA: your friend has nothing to feel guilty about, she took her animal to the vets and trusted their 'expert opinion', she can do nothing more. Granted a second opinion could have been looked for sooner, but as above, she is not a vet and trusted her vets opinion. Is she getting a new vet!?