I agree with the others. Start slow with maybe 30 min sessions and then increase the length and intensity once the client feels stronger and more confident to do more. Keep talking to them and always check to see how they are feeling during and after each workout and then you can decide for the next one. Talking to their physician is also a good idea, but the best source of information is your client himself/herself. Sometimes a simple walk can provide you with a lot of useful information.
I agree with Paul and Karin on this. Remember exercise can effect the immune system both positive and negatively. If you do too much you can deplete the immune system making it more likely they are sick again. Go easy with them. No interval training/HITT.
Stick with low intensity cardio, use the RPE scale and Talk Test and do circuit training for resistance. Start with at most 30 min exercise and no more. Increase the time of exercise before the intensity of exercise.
this is personal advice. I had pneumonia 25 years ago, and it was a humbling experience. My first day in the gym, I did 15 minutes of cardio, felt reasonably well and decided to finish a few minutes in the hot tub. I fainted!
My suggestion is to start with too little. Stop when your client feels that s/he can still do more and remind the client that tomorrow is another day.
After that I would just do some stretching and end the first few sessions after 30 minutes. I would never have believed how much that lenses took out of me.
I would get those suggestions right from the horse’s mouth – the physician who cleared him/her. They’ll be glad to talk to you, plus it’s a great opportunity for you to introduce yourself to that physician, who could possibly become a nice source of referrals for you – you never know.