We always advise that all important communication is undertaken in writing - phone calls are easily denied afterwards. Send a copy of the decision letter to your adviser (and keep hold of the proof of postage).
Nothing on this board constitutes legal advice - always consult a professional about specific problems
So if I wrote a letter, how formal do I have to be ? As I say, I got on quite well with my adviser and we would chat quite freely, although of course it is hard to tell if they even remember people individually due to the mass they have to see.
I would go for 'semi formal' but it is up to you. You need to draw his attention to the decision letter and say that, for this reason, you will not be attending the next WFI.
Nothing on this board constitutes legal advice - always consult a professional about specific problems