In reply to susipeg:
I'm not a mental health expert, but a fairly lowly vet. We do, though, deal with a lot of behavioural issues and states which, by necessity, require management through non-talking methods. And the common thread in any approach is to recognise that whilst therapy from an experienced professional is key to success, so is creating awareness of the patient's understanding of the world amongst those closest to him/her.
You're already a good friend by virtue of your wish to help. One of the best steps you could take next would be to get really clued up on what makes your friend tick. You might already have a diagnosis on which to research further; or you might be working in the dark. If the latter, a useful start would be to look at the following:
1) All cats have Asperger Syndrome - this is actually a picture book, written for kids whose siblings have Aspergers, which explains why they behave as they do, but from a position of describing 'normal' cat behaviour, as it's analogous in many ways. It's available to buy, but there are usually some slide shows on YouTube involving scanned copies. It only takes about five minutes to read.
2) Slightly more involved is Pathological Demand Avoidance Syndrome, which is characterised by extreme behaviour when sufferers are placed under stress. Its position as a separate syndrome is controversial, but it's a pattern of behaviour which can be recognisable in many on the autistic spectrum and an understanding of the triggers can help you to help your friend to avoid stressors.
Your friend may not display any characteristics described in the above situations, but they are a good place to start if you want to understand some of the difficulties s/he may face with the world. But having a more concrete diagnosis to go on would help immeasurably and for that you obviously need assessment.
Martin