Do I need to sever joint tenancy if the other named person on deeds has done this?

Writing this for a friend.

My friend used her money from a previous property to buy a new house with a partner to live in with their child. Unfortunately she named him on the deeds and on the mortgage at the time. He contributed nothing to the deposit and only contributed to the mortgage (on which he is still jointly named despite her trying to have him removed) for 6 months before leaving my friend and their child. Now the child is over 18 years old, he is trying to claim half the property despite having never contributed to the maintenance or paid anything towards the mortgage except for the first 6 months of them living there. 

She has received a solicitors letter informing her of his desire to sever the joint tenancy and there have been previous letters stating his intention to force the sale of the house that she and her child still live in. 

My friend was in the process of completing the severance of tenancy when she received the same from his lawyer. The question is, does my friend still need to complete the SEV form and notify him? Is there a way of checking the status of the title deeds? Will she be informed by post that they are now tenants in common?

Cindy - if a joint owner severs to joint tenancy, and applies to register a form A/joint ownership restriction, then there's no need for the other owner to do the same as we can only register the one form A restriction
You can search online for a fee at any time re the register - Search for land and property information - GOV.UK 
You can use the free Property Alert service to be notified of a significant application being submitted - Property Alert - GOV.UK 
We may serve notice on the other joint owner but that's a caseworker consideration based on what has been submitted with the form SEV
I would strongly recommend seeking legal advice with regards the impact of such a severance and registration of a form A restriction. I'm sure you have already re the claims being made re the property but it's important to understand the impact or otherwise of a form A restriction and how it impacts. Joint property ownership: Overview - GOV.UK