My name is XXXXX XXXXX I'd be happy to answer your questions today. I'm sorry to hear that this happened.
The problem here is, the jury is free to decide which witnesses to believe and which witnesses not to believe. Thus, on appeal, the court
looks not at whether any particular piece of evidence was accurate, but simply whether there was sufficient evidence which, if believed, could support the conviction. They cannot weigh the evidence, because they weren't there.
What does that mean? Say I'm in a car accident, and I bring in a convicted child molester to testify that the light was green and I had the right of way. The other side brings in 100 nuns to testify that HIS light was green and HE actually had the right of way. The jury could choose to believe the child molester and disbelieve all 100 nuns. On appeal, I would win, because there is evidence to support the jury's finding.
In your case, if there was a witness to the fight that identified the assailant, and a police officer testified to the same thing, that is enough to support the conviction. The fact that other witnesses couldn't identify the defendant doesn't necessarily mean that it wasn't him, and a jury can choose to believe a police officer over the defendant's girlfriend, because she'll be considered to be biased.
Unfortunately, that means that an appeal on the grounds of lack of evidence would likely not be successful. That doesn't mean that he shouldn't pursue it - a criminal
defendant has a right to follow all avenues of appeal, and is allowed at least one appeal as a matter of right. Just don't get your hopes up too much.
I apologize that this was probably not the Answer you were hoping to receive. However, it would be unfair to you and unprofessional of me were I to provide you with anything less than truthful and honest information. I hope you understand.