Rodney Yee is a teacher in his late 40s with tons of videos. He is an excellent teacher, very comfortable with on screen teaching. Anything by him would be good, however a lot of it is challenging. There is one I have with a 4 or 5 short practices which range from easy to difficult, but I cannot find it in the pile on my desk to give you a title.
Lilias Folan would be great. She actually trained with Vishnu Devananda, and went on TV in 1970…. I can remember doing yoga watching her when I was ten or eleven.
You could try the Kripalu shop online. The Kripalu staff offer videos in various categories: gentle, moderate, vigorous. Think about the level your client would be most happy with. I have stayed several times at the Kripalu center, and their teachers are good and well trained, and the videos produced by Kripalu will feature those teachers.
The question is not just the age, but the fitness level, flexibility, underlaying health issues, and so on….. it is hard to make a judgement without having the person on the mat in front of me…. There are huge differences between styles and between teachers.. sometimes you have to try a few to see what speaks to you.
Generally I would say unless they are VERY fit to avoid anything called Power, or Ashtanga, or Bikram to start. Not that you cannot do those at 50. My sister is 62 and does Ashtanga twice a week, but to start it can be challenging.
The other thing is that a video is really helpful if getting to a teacher does not work, but part of what makes yoga what it is is the connections that are forged by the interactions of the teacher and student.
I looked up your web site (very nice btw… I taught at Sport and Health in Tenley in the 90s) and would say in your area there are tons and tons of yoga studios with great classes. If your client likes the video you give her, you might want to recommend she try going to do it face to face with a teacher, just to get a sense of what that is like.