Ah, the totally clueless emotional vampire with no boundaries.
On one hand, you tend to feel kind of sorry for people like this because they are obviously lonely and pathetic. She has no real friends, so she has to rely on manipulation for attention.
However, they will take and take and take until you have nothing left, and will never give anything in return. She shows the classic signs - she phones with 'emergencies' carefully crafted for times when it's harder to brush her off (just after the buses stop, or on weekends). She overstates situations to get immediate attention. She's careful to interact with the people who will give her the best response. She uses guilt - the ER calls have you thinking "Well, what if it is a real emergency" or "What about her son", plus the Church Friends card ("if you were a real Christian you'd help me). She puts herself into positions where she'll be in real difficulty if no-one steps in (like showing up with no ride home) when she could as easily have phoned before the event.
With people like this you need to set firm limits - the next time she phones for a ride, tell her that you will phone either a cab or an ambulance for her. Or, if you can't bring yourself to do that *you* take her (not your husband) and drop her *and* her kid off at the entrance and leave. With no one to flirt with and a cranky small kid the ER will look a lot less inviting.
You can't stop her from being an emotional vampire, but you can stop her from feeding off of you.