Supersession for PIPs

Hi, I would be grateful for some advice re requesting a supersession for PIPs as I've searched the site and can find very little information.

In 2015, a close family friend (now aged 27) was awarded enhanced living PIPs but denied any mobility payments (scored 4) despite being born with a debilitating condition which necessitated endless hospital admissions to Great Ormand Street Hospital throughout his childhood and into adulthood, numerous operations (including recent removal of a kidney) and having severe muscular skeletal problems with obvious deformities. His mobility is severely limited due to a combination of: flat feet, knock knees, hyper-mobility, low muscle tone, joint pain in ankles, knees and hips, fibromyalgia, permanently dislocated elbow and shoulder joints etc etc etc. with dizziness and chronic fatigue. He is in the support group for ESA, which was granted without any difficulty. During his original assessment for PIPs in 2015 he didn't mention the pain and severe exhaustion mobility causes him as this is something he has lived with all his life and he remains resolutely positive and is extremely stoic and brave. Instead, he underplayed his condition (as he always does) and didn't realise the importance of stressing the difficulties, pain and exhaustion he faces on a daily basis as he assumed this was obvious to the assessor! He now realises he should have appealed the decision at the time but was in and out of hospital, culminating in having his kidney removed, and suffering from exhaustion which left him unable to do so. It is obviously way past the deadline to appeal this decision now.

However, I think he may have grounds for requesting a supersession because his condition has deteriorated, his pain and fatigue has increased and he recently got a new diagnosis that his health problems are actually all caused by an extremely rare genetic syndrome (150 cases worldwide). This condition is known to be degenerative.

I would like to know how great the changes have to be for the DWP to grant a supersession and how he should word his changes. Furthermore, I would be grateful for any advice regarding the following questions:

1) If he contacts the DWP to report a change of circumstances should this be done by phone, email or snailmail?
2) Should he mention he wants a supersession and should he state his new diagnosis and detail the deterioration in his condition or just ask for forms to be sent to him?
3) Should he include any medical evidence at this stage?
4) Would telling the DWP of his change of circumstances automatically trigger a review of his case?
5) Would he have to answer the phone questions again to get any necessary forms then complete the original PIPs assessment forms again?
6) Would his current enhanced daily living PIPs continue to be paid while his case was being reviewed or would payments be frozen in the interim?
7) What is the risk that by reporting this change he could lose PIPs altogether?
8) Is he likely to have another face to face interview?
9) How long is the process likely to take?

Thanking you so much for taking the time to read this.

Thank you so much for your reply Gordon. Can I just clarify your answer to my question 6 please...
6) Would his current enhanced daily living PIPs continue to be paid while his case was being reviewed or would payments be frozen in the interim?

You answered 'yes' to this, do you mean yes his current rate of PIPs would continue to be paid or yes payments would be frozen I.e, stop while his case is reconsidered?

Also, you said he should write that his condition has deteriorated, should he explain in what way and give his new diagnosis in the letter or wait for the interview?
Thanks again