I would suggest dampening down the emotion, keep it more factually related. If he wants to add a brief statement about how this made him 'feel' it could probably best be handled with a quick, one-line statement in the closing paragraph. Terms like 'cheated' may not elicit a great response, but I think it would be fair for your friend to say that he was frusturated and left confused by the contradictory information provided and that he would not expect this type of customer service from United.

In addition, I'm not sure what he wants from all this, but getting a form-letter apology 12 weeks from now is probably not it. Have him tell United what his expectations are - miles, upgrade certs, etc.