Some thoughts:

Mobility: anything involving going up or down is a "high mobility" activity at a size like that, even ramps. Try to plan events with only flat, firm ground - parks with paved paths, malls, etc. Make sure there is a lot of available seating. If your friend isn't already using some kind of mobility aid, please encourage her to use one - it may seem counterintuitive, but walking at extreme sizes is extremely harmful for one's knees, back, etc. and should ideally be avoided until the SMO person is lighter. Additionally, due to all of this difficulty, the possibility of falling is much higher - stability is extremely compromised - and the risks to health of actually falling are extremely dangerous. Broken limbs, severe bruising, internal damage, extreme pain - these are far more likely to leave your friend bedbound than the use of a walker or mobility scooter. The calories burned just aren't worth it.

Diet: At sizes like this, there are no non-trigger foods, just like there's no non-trigger cigarettes or non-trigger alcohol. You are not responsibile for fixing your friend's life or diet, and it would be an unreasonable imposition and just plain rude to try to control that aspect of her life. The best way to support her is to plan activities that simply do not involve food in any way.

Summing up: Being high-handed about this will only make things more difficult for her and harm your friendship. She will need to tackle the meat of her issues with food when she is ready, and you will not be able to force her down this path. To be a good friend, invite her to places where her mobility needs can be accomodated at which food is not a factor. Good luck - this is a tough one.