Sorry mate, this seems like a pretty clear no-go in my book. I couldn't care less about the age difference. Some people may not like it but she's old enough to make her own decisions and manage the ups and downs as they come. However, she is in a relationship. As a general rule of thumb, thats a pretty big "nope" flag, heightened in this case by the fact that you have little (if any) reason to believe that she will leave him for you, and indeed have good reason to believe theyll continue to be together for a good while. Obviously theres issue with going after someone who is taken, but skimming over that, youre just setting yourself up to get emotionally screwed over, more so than you may already be.

Side bit, has she expressed any interest past friendship with you? It seems clear that she isnt likely to leave her existing relationship for you, but Im a little uncertain as to if shes actually expressed any interest in you, outside of that. If not, I feel it only adds weight to my words.