While "affairs of the heart" and matters of love and affection are far from being my specialized knowledge (just ask my wife), I believe you are correct. History is full of stories of soldiers, cops etc. marrying the wives of fallen comrades. I believe there were at least a couple (and maybe several) firefighters who marred the widows of 9/11 firefighters.
Those firefighters left their own wives to marry those widows. There was a television special about it. It is some psychological phenomenon with a name I can't recall.
If one truly loves one's spouse and wishes only their happiness and one has chosen one's friends carefully and with love, it seems natural to me that they should have a connection through you and a responsibility toward one another.
Frankly, if someone does not trust their best friend to behave with honor and love towards themselves and their family, they should review who they choose as a friend.
I wish this family much love and joy and I honor their sacrifice, as they have honored Captain Scott Speicher's.
My brother-in-law is a cop. One of his good friends was killed while on duty. His young wife went on to marry another cop who was also a friend of her deceased husband. One of the young guys who worked for my husband was killed in a car accident and his wife married another sailor that was a close friend of her husband. As you said, it happens all the time and I don't think there is anything wrong with it.