How do you know if your friend can keep commitments or not ?

everytime i tell my friend to meet me somewhere she never comes at the time we agreed upon and she does this constantly and it irritates me alot because she has the tendencey to blame it on her mom saying that "ooo my moma be tripping you know how a @#*#! she is " and i know she knows its statring to irritate me and if she can't keep the commitment she might aswell not make one i mean does she do this to irritate me or can i not trust her with being where i want her to be on time