I'm not addressing his ex-gf problem here because I don't know the terms of their relationship and really don't give a sh!t about it either.
Calling a girl you've only known for a little bit that you've banged for the first time is another story. It depends on the situation, her personality, and how hot she is. Most of you already know the rules to this basic logic sh!t so this is a pointless post I'm making here, but for people like Matt who don't...
Unless she is a real sweet heart and isn't the type of chick who tries to countergame your game (very rare), don't call her for a few days. A lot of b!tches like to test you and see if they've "got you" right after giving it up. If she ends up calling me, I tell her I'm too busy for at least two or three days before we meet up again so that she's craving me and realizes she isn't the center of my world all of a sudden just because she gave me some a$$. Also, the hotter she is, the longer you have to make her wait. (You can figure out why.)
Exceptions to all of this are when she is literally begging for the d!ck over the phone...or when she actually drives over without telling you she's coming by, and throws herself at you. How you react in those circumstances is completely up to you.
Keep in mind that there's a HUGE difference between the ones ACTING like they are offended by you not calling them (which is the case 90% of the time), and the ones that get SERIOUSLY offended. You should be able to figure out WAY BEFORE you've banged her which part of the spectrum she falls into, but 9 times out of 10 you're better off giving her a chase.
By the way, this is not a question of maturity vs immaturity. This is the way hoes work. Less is always more for them. It may be pathetic, but it's a necessary evil if you want to keep her interested.