Her boyfriend is secretly jealous that she will have a better phone than him so he is working hard to convince her to get that Hero.
Tell her to get the Evo... this is a good test to see how he reacts when his lady is sporting better hardware than him...
EDIT: came back to post something useful
The main differences are below:
Larger 4.3" display (much easier typing and viewing videos)
Faster CPU (this is especially noticeable when actively using the phone over a period of time)
Better rear camera (8MP vs. 5MP)
Front-facing camera (She can take great pics of herself)
HDMI out (she might never use it)
Much greater re-sale value (if she is that type)
Updates will be supported for another few months (Hero is all updated out)