property law helpWatch
I would say it doesn't meet the requirements, because it is not clear that he/she wishes to sever the joint tenancy. Also, as he didn't serve notice on all of the tenants, it would be worrisome to rely on second hand notice. Just my opinion.
Personally I believe that there is evidence of an intention to sever. But that's just my view.
You need to share your own thoughts and remember that you will only get credit if you show your reasoning and support it with authority.
Good luck with it.