Can anyone recommend what the best special effects software is to make a person look thinner in a partcular scene? And what this would entail? Thanks

What the "best" software is is always a subjective question, and the effect you're trying to achieve can likely be done many ways. However, off the top of my head I'd say you could probably get decent results with after effects, and this tutorial (http://www.videocopilot.net/tutorials/demon_face_warp/) should help with the basic technique involved.

The best possible way, in my opinion, is to have this person loose the amount of weight needed to make the scene work (in a safe and informed manner, mind you). Why do something with 'special effects' if you can do it better without?

Depends on the desired effect... the OP could be trying to achieve something like what they did in the film Thinner, which is what I took from the post.