defenitly, not even walking. to much stress, to much force. also, usually when someone is that obese, they will have altered gait and muscle imbalances because of it. The bike is the safest bet. Get them on an upright bike, sitting straigh up (not leaning down into a race type of position).
I hear ya about not faking it on the bike. But, as you will see when you start your job as a trainer, you need to know how to get the most out of your client. As far as figuring out where they are cardio wise here is what I do for someone that deconditioned on the bike to make sure I know they are working.
1) I test set them up in comfortable position on the bike.
2) get a heart rate monitor on them
3) have them start pedaling and note their heart rate
4) every 2min. increase the level on them and note their heart rate and where it stabalizes.
Now, here is where you find out if they are working. You need to keep talking to them. Remeber, that real steady state cardio (aerobic cardio) is done at a pace that you can maintain a conversation without getting out of breathe. So, as you are talking to them notice how they are breathing. Once they start panting to keep up with your conversation note their heart rate and stop the test and move on. Now, you have the number (generically but it is something to base your training off of) that they go anerobic at. So, you will simply set them up with intervals on their workouts were they are to pedal up to 2-5 beats from that 'anerobic' heart rate number and then take as much rest as they need to get back down about 20bpms from that 'anerobic' number and then perform another interval again up to 2-5 bpms of that 'anerobic' heart rate. As they increase their work capacity, the time between intervals will decrease as their recovery gets better (that is real improvement!). Also, they wont be as winded when they approach that 'anerobic' number and they will start to be able to exceed it. Around 4-6 weeks you can retest and then get a new baseline.
hope that helps.