Why does the texting go hot and cold.....?

Why does this girl text him one minute then disappear for half the day then text him again and she never seems to want to speak by phone. We live together, it's his house, but this is my home too and i don't have any other options where i can go live. This 24 yr. old is interfering terribly with my life, mylivilihood, my schooling, my kids, MOST OF ALL MY MAN!!!!! I really think he is just infatuated with her cuz she is young and skinny (well, i'm very thin also). I'm not willing to let her take MY LIFE away from me here!!! She is always keeping him hanging by a thread and he doesn't know why so he said the next time he sees her he is going o ask her what's up with the hot and cold text messages cuz it's getting on his nerves and he HATES texting anyway (always has!) I think she does texting cuz she's young and the young people are into texting? She also likes to go to the bars. He DOES NOT DRINK. He seems fine with her bar habits now but i'm thinking he is not going to sty fine with it. He sayd that there still may be a chance for me and him again so i need to make sure that happens but how?!!! HELP ME PLEASE!!!!!