Coming in late to this conversation:
How could TonE have mitigated the situation though--other than being a different person? He's the typical male student of the DJ variety, away from his hometown, living with a bushel of roommates, playing video games, allegedly smoking the ganja, taking night classes, having a weird schedule, and so on. Casey is the chameleon saying everything he wants to hear and doing everything he wants to do. She was not honest with him about her job (or lack of one), her relationship with her parents, her other ongoing physical relationships, that she was stealing to finance their meals, or even what she wanted from him. Casey was coming from a place of dishonesty that was not bred in her by TonE (as we know she lied to Jessie, Ricardo, and so on).
So, we have Casey trying desperately to fit his square peg in her round hole so to speak. He's just cruising along like many young males his age do, like you said getting "something for nothing", but he seemingly was honest about some major facts like not wanting a child around partying, in the bed with them at night, and so on. To me, that is a healthier approach than having the child around partying or seeing non-parental (or parental--gag) sex in the bed. Further, he was letting Casey know in that immature male way that he was not that into her. Did he come out and spell it out for her? Not that I know of. But anyone with logic and common sense can read between the lines of their text messages and see that he did not love her and she tried to use sex and jealousy to manipulate him (did not work too well). Should he not have engaged with her when he knew it wasn't likely to go further? Perhaps. I bet he won't make that mistake again!
He knew she had a child before he "dated" her so it could not be Caylee that really got in the way. Perhaps she moved too soon and too fast for his liking. Does that make him "guilty" because he did not want Caylee at his pad but was willing to take Casey in? I'm sure he had no clue about her InvisiNanny ruse. Was he supposed to be so principaled as to send her home to be with her child? Perhaps.
If he's guilty for simply sticking his square peg in her round hole then there are also a lot of men around O-Town with the same guilt.
To me, if we are going to ration guilt around I can find two other people closer to her who bear more the weight of it (in my mind) that the luckless TonE.
Just my .02.