We have to begin by stating the obvious: Susan is a teenage girl whose boyfriend committed suicide. Nothing is easy after that.
Susan is yet another female in this novel suffering from low self-esteem and using her sexuality to cope with it: "She got a little taller and prettier and grew breasts. Now, she acts a lot dumber in the hallways, especially when boys are around" (1.2.2). What's with all these women trying to impress the not-impress-worthy guys around them? Is there something in the water, or is it just high school?
Here's the thing: Charlie never once thinks about how Michael's suicide may have affected her, or how she's coping with it. All he gives her is a passive-aggressive confrontation: "Do you ever miss him?" (4.1.18). Maybe Susan could use a friend just as much as Charlie.
Charlie's conclusion? "If Michael were still around, Susan probably wouldn't be 'going out' with him anymore. Not because she's a bad person, or shallow or mean. But because things change. And friends leave. And life doesn't stop for anybody" (4.1.21).
We might be inclined to believe Charlie if he ever got to know her, but—surprise, surprise—he didn't.