How do I set boundaries with my friend who has a TBI

A fellow caregiver asked...

I'm having trouble setting caregiving/time boundaries with my friend. She is unable to drive because of her head injury and often asks me to help her run errands. Most errands I don't mind, but lately, it feels like she's taking advantage. Recently, I showed up to pick her up for one errand and she had two other disabled people, who had been dropped off at her house, waiting to come along with us. She had invited them without even asking me. It was a very uncomfortable situation, because they couldn't be left there alone. I felt my only choice was to kindly take them or make a scene by saying no and waiting for hours for their caregivers to come pick them up. The day was absolutely miserable for me and gently I let her know afterwards that I wasn't comfortable with bringing extra people along. Additionally, the errands she asks of me always start out simple, but become very convoluted and inconvenient. She might need a specific service, but instead of choosing someone local to make the trip convenient for me, she'll make an appointment with someone an hour away (it's already 60 minutes round trip to her house and back). I'm sure people reading this are thinking, "Why don't you just say no?" I've tried saying a very kind no in the past and she uses emotional blackmail to get me to give in. For example. she asked me one evening to take her to a two-hour support group meeting. I already had important family plans, so I apologized and said that I couldn't take her. Instead of accepting my no, she went crazy; crying and threatening to kill herself. I ended up dropping my plans and taking her to the meeting. Luckily, while there, I found that there was a ride service she could use. I found out she knew about the service, but preferred going with me.
I feel badly that she doesn't have good family support and that her partner isn't helpful to her. I have one day per week (6-12 hours) that I usually spend with her helping her out. It's gotten to the point that I dread that day. She tries to get me to do activities with her on other days, too, and I usually stand firm with my own plans. I''ve learned that her invitation to a "fun" activity is usually a ruse for a ride to something that I find miserable. I've attempted to find services for her, but most cost money because she has a great deal of assets. She wants help that is free and even says it. This person has the money to call a cab or take another form of public transportation, but will gladly save money by getting someone to do errands for her for free. I'm trying to distance myself from her and break things off without blow ups or her harming herself. Anyone else have a situation like this?