I was a volunteer hospital car driver 3 days a week for 10 years. My car would ping if the seat belts were not used, and so would allow me to say, are you having problems shutting your seat belt ? I can come and do it for you if you need some help. Only had one person state that they would not wear one and did not have a letter which excused them from wearing one , and I simply turned the engine off and said I am afraid I am not allowed to take you unless you use your seat belt. If you dont use it I am unable to take you to your appointment this morning. He huffed an puffed a bit but put it on. On the way back he again got in without putting his belt on and I asked if he wanted to come with me or would he like directions to the nearest bus stop. As he lived over 40 miles from the hospital, he begrudgingly and with much muttering put it on, then a little later took it off, but of course my dashboard showed that it was not on and it pinged so I just pulled up and turned the engine off and said not a word, just sat there. Another passenger asked why have we stopped? I replied we are waiting for Mr X and he huffed a bit but put it back on and I switched the engine on and without a word carried on! Regarding the person who would not wear a seat belt with you, you could either wait until it happened again and then either appeal to the driver, to explain the law to her, or say I am sorry I will have to go on my own and get out. alternatively you could preempt that by when you next see her saying in a reasonable tone, how sorry you are that you will not be able to travel together any more, but you will be happy to walk with her when that is convenient. That way you show your determination not to be in such a situation again, but leave it up to her to make her own mind up