Hey Julie,
There are a few things you need to remember when working with severely obese clients (clients who need to lose more that 150 pounds of fat). Here are some tips that may help you.
1) A max is a max-Obese clients are no different than regular clients if you look at thing in the eye of max effort. Yes there max effort is going to be far less than someone else's but this is what you are trying to achieve none the less. You have to be careful and monitor more, but don't be afraid to push boundaries either. As far as movements I like...
Alternating leg Aerobic Step ups-Now these are step ups where you are going to need to push off with your other foot. Keep the height low to start, raise the height as she progresses.
High Leg Side Steps-Think of the start of a sumo side lunge. It basically is one just with no lowering of the body. Very effective and allows some different plane movements
Plank Ups-No obviously holding up the body weight for an extended period of time isn't possible for these clients, but the goal is to work it towards that. For this I like plank ups. You get in a prone position and raise into a plank and then lower, much like push ups but the elbow raise support allows for the client to not strain their wrists and feel more stable. Over time you can progress how long they hold at the top. move from 2 secs to 3 secs, etc. (this is the prone posititon I am speaking of...
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Toe Touches/DB Deadlifts-Just simple toe touches do wonders, you can progress to DB deadlifts.
So that some movement.
2) Diet is everything-Now you may not be a nutritionists, but if you are talking to them about their diet, your wasting your time. Someone of that level of body fat has severe problems with food and horrible dietary habits. You need to provide them with some little guide or point them to one you support that will help them with their diet.
3) Psychology 101-Usually people in this area are really going to need a friend and someone to take them seriously. This is a hard change, when some of us think about the frustration of losing 20 pounds, try 150 or more. Then have someone tell you it slow and steady that wins the race. Motivation, praise, and constant education is key. Never assume they understand anything, overly explain thing, make it impossible for them to think in their head "well i just didn't know". Take away the excuses and make it clear they are making the choice to not do what they need to do.
Hope that helps, good luck to you and your client Julie.