In Jamaica the Dunn's River Falls trip is amenable to "Observers" but you do still have to pay since the fare includes the bus ride and admission to the park. The good news is that there is a wooden staircase all along the falls where you can see pretty much everything without getting wet. (I think the top and the bottom can even be accessed without climbing stairs.) You can even access the various pools along the way without climbing the falls themselves.
My brother-in-law did this. He didn't want to climb the falls, but he was able to be "with" us all the way up, taking pictures, shouting out to us, etc., and watching his daughters and wife climb the falls and sharing in their smiles and laughter. It was worth the price of admission to him.
This could be a good option is some of your group want something active and others just want to watch. Likewise, if anyone in your group isn't sure if they can handle climbing the falls, they can opt out at multiple spots along the way. Having said that, my 70-year-old mother made it all the way up. The guides were very attentive and encouraging to her, and she inspired others in our group who were uncertain. She was about to opt out of the final stretch, but the guide said to her, "you've already done the hardest part!" so she went on and finished it.