Personally, I wouldn’t *just* let them do the exercise anyway if they are really uncomfortable and unable to navigate the machine. Point being if they can’t sit properly (while you don’t elaborate I’m assuming this means their lumbar spine and posture is poor), there’s too much risk of injury! Especially with seniors. However, you don’t mention anything specific about the client’s age or background.
It’s really easy to have them sit it a chair in GOOD posture, and use your hand behind their heel as resistance the same way the machine would. Men like to “muscle through” exercises with poor posture but there’s finesse to getting around that too.
If he’s been taking Yoga 5 years and isn’t getting hamstring relief, I would look at other muscular dysfunction and greater frequency of stretching. It doesn’t mean it’s a “bad” Yoga class or that they aren’t working on hamstrings.
Then, some folks just have very tight hamstrings regardless of what they do to help it. Stretches don’t correct dysfunction forever. Research shows for really, only an hour. Especially if the client is sitting a lot.
You could try foam rolling before exercise, specifically hamstrings, but I would NOT use static stretching with a rope. Useless as a warm up. I’d likely add some functional drills like walkouts, SL RDLs or even frankenstein walking to warm up & use dynamic drills to raise awareness.
Hope that helps. Best,