My friend says he feel permanent numbness in different parts of his body skin like in his arms, legs, etc. and thinks his skin is slowly getting numb. is this a serious case? plus he have a brown vertical line in his right thumb. what could this be?

Numbness: Numbness is a serious issue. Would follow up with neurology could be lyme, or from diabetes, or low B12-especially if vegan, other vitamins deficiency, or alcohol( usually older) or radiculopathy -nerve compression if had significant sports history, or autoimmune disease or MS. Follow up with MD/ urgent care/ ER this week for referral.