The Tuscan, by the name of Davide, is one of many Italians who have ventured abroad to legally end their life in recent times.
Having been diagnosed in 1993, Davide has found his condition to be unbearable over the past few years.
MS is a chronic disease that attacks the central nervous system which can result in vision problems, tingling and numbness, pain and spasms, fatigue and weakness.
"I have arrived in Switzerland with Davide who asked to be able to choose his death, opportunely, because he couldn't take it any more after so many years of suffering," Welby said in a video message.
Welby is the widow of Piergiorgio Welby, an Italian poet, painter and activist whose three-month-long battle to establish his right to die in 2006 led to a debate about euthanasia in Italy, rekindled by DJ Fabo's death and other assisted suicides.
A bill on end-of-life issues including living wills, but not euthanasia, is before parliament amid criticism from the Catholic Church and conservative politicians.
