ya i totally understand.
i had (note the HAD) a friend who i always there for - i didn't have any kids and wasn't married yet so it was easy for me to go and pick up her kids or take her son to hockey when she took the daughters to dance or to have her drop them off at my house for a sleep over or an afternoon in the pool but when i got married and got paregnant she seemed to never be home to answer my calls.
i too would leave a message and wait up to a week for a response.
i never asked her to watch my daughter but it was like now i couldn't (or wouldn't) be at her beck and call she didn't have time for me.
dd is almost 4 and i haven't seen her exfriend since about 2 months after dd was born. she even blew off dd's christening even though i talked to her that morning and she told me she was on her way - a month later she sent me an email saying sorry something came up.
i dont' have time for that crap anymore - i put up with it when i was single and no family but once my dd came in to my life i won't put up with it anymore.
dh has a friend like that too. and we haven't seen much of him either - i think they get together once a year for coffee but that's it.
if it bothers you then you should probably end it or it will continue to bug you. i'm sure they'll continue to be friends at school.
it's too bad some people think the world is all about them

sorry she's so selfish but she probably won't change