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.