It all depends on the degree of scoliosis as Heike mentioned. And it also depends on the fitness level of your client. Some people respond very differently on exercises that others do. It’s difficult to give you a definite answer to your question because I have not assessed your client. I could say, yes it’s ok for the client to do lat pull-downs, but it won’t be the right thing to do.
I know this is not the answer you are looking for, but I would also recommend swimming for many obvious reasons. For dry land exercises and strength training, you should also consider exercises with tubing because they are not as “intense” or invasive for someone with scoliosis as any of the other strength training equipment you find in the gym. You definitely don’t want to do exercises that put stress on the spine (overhead presses, squats with barbells, ect.) I checked your profile and it shows that you have been a PT for many years now. I’m sure there are a number of exercises you can incorporate from that field into the fitness training. I hope this helps.