If you leave you friend in the lurch, you've left her in a bad situation. If she was counting on a ride home after school and you forgot to wait for her, you left her in the lurch.
When someone is left in the lurch, she's left needing help of some kind, usually abruptly and surprisingly. It's the perfect phrase to use when a person is counting on assistance that doesn't come. This kind of lurch has an entirely different root from the lurch meaning "abrupt roll or tilt." This lurch comes from a sixteenth century game, lourche, and the situation in which one player loses by a wide margin — or is left in the lurch by the other.