can someone explain this to me?

so my homegirl

I posted about her before
http://board.okayplayer.com/okp.php?az=show_topic&forum=4&topic_id=12745559&mesg_id=12745559&listing_type=search


since the texting naked pic incident she has supposedly been "done" with her man at least four times.

Lowlights
-2weeks ago she says she's done with him cause he ain't check for her since she returned from vacation
-3 days later they are back together strolling the boardwalk

-two days after that she's done with him because apparently back when the text situation happened he offered to change his number and still hadn't she gave him an ultimatum he said he's changed his mind and isn't going to do it she says she's done

-a day after this ^ they chilling watching movies together again

-during this^ his phone rings and just a number shows
he doesn't answer it
she asks him why he isn't answering
he says he doesn't answer numbers he doesn't know
she looks at the number and realizes it's naked chicks number
she looks in his phone and see that she's messaged him
"hey it's been a while how r u? when r u goin to see me"
his response "I've been busy with work"

obviously my homegirl is pissed cause he didn't tell naked girl that he has a girlfriend

thats when she hits me with the "I'm so mad..but I need proof you know.. like I have to catch him so that I know he's cheating for real"

my response :/ you sound likea girl on maury


can someone explain to me the need for 'concrete-catch him with his pants down" proof?

additional question do ya'll think he's cheating?