Late for a Very Important Date

I have a situation that I would like a little advice about.

There's another mom at school whom I've become friends with due to the similarity of our children's ages.  We both have a 9th grader, a 7th grader, and a 4th grader.  I have a 5 year old and she has two more younger children.  We often joke about how crazy we both are for having so many kids and I tease her more for having five children.  She's a good Christian mom and I like & trust her husband too.  I've allowed Big Sis to attend functions at their house, which is not something I do with very many school friends.

At some point this year, we discussed that we should consider car pooling since we both have 3 kids who are in 3 different schools.  We live about a half mile from each other, so it wouldn't be too inconvenient for either of us.  Time passed; nothing happened.  She brought it up again before Christmas, so we decided to give it a try.  It was around the same time I found out I was going to start babysitting a little Autistic boy in our church.  I was supposed to have him on Wednesdays and Thursdays so pick up on those days would be very difficult anyhow.  We agreed that I would do Monday and Tuesday, she would do Wednesday and Thursday, and we would alternate Fridays.

So far, it's gone fairly well.  She's always followed through when it's her assigned day and is flexible to any change in schedule I have to make.  There's one problem, however.  Her daughter is in NO hurry to get to the car.  My son passes her on his way out the door every day while she stands and talks to her friends at her locker.  He reaches the car and then we have to wait an extra five to ten minutes on her to arrive.  When we do see her coming, she's walking as slow as molasses, looking around like she's out for a Sunday stroll.  I thought that she would try to hurry a bit when she realized we waited on her every day.  I was wrong.  I also thought that maybe her mom would realize that my son makes it to the car so much faster and would question her daughter's lateness.  Nope.

Last week, Hubby was doing pick up for me and finally called me.  "Are we picking up this girl?" he asked.  I confirmed that yes, he was supposed to pick her up.  My two boys and her brother were both already in the car.  I told Hubby I'd text her mom and make sure she didn't go home from school sick.  "Is 'Morgan' at school today?" I texted, "Because she's still not out."  A couple minutes later, Hubby replied that he finally saw her coming (walking like a Senior Citizen).  "Never mind," I replied to her mom, "Here she comes."  I didn't feel bad about it because she needed to know we were getting irritated but I didn't want to say anything.

The next day, Son #1 was walking out of the building and he passed her.  She was at a friend's locker, chatting, but she stopped him and asked which mom was picking up.  He said it was our turn.  She arrived at the car several minutes later and said to Hubby, "Sorry it takes me so long to get to the car...I have a lot of places I have to stop."  So obviously her mom said something to her.  Yet, she's lying about having obligations to fulfill.  The places she needs to stop are to see friends.  I find that very annoying.  School traffic is insane and we would like to get out of there as soon as possible.

In my opinion, we have two options.  We can either let it go and do nothing.  OR- we can tell her mom that we're going to quit the carpool and pick up only our kids from now on.  Either option is inconvenient for us.  But I've already said one thing to her mom and I don't feel like I can bring it up again.  I feel like this girl is being very rude and inconsiderate of our time.  What would you do?