Q. “He wanted to see his friend John Smith before going out”? Or “He wanted to see his friend, John Smith, before going out”? I know the rule for spouses and siblings (as in CMOS 6.31), but what about friends?
A. Because people usually have more than one friend, the first version, the one without commas, will almost always be correct. Even those who might claim to have just one friend could make another one at any time. Besides, friend is an elastic concept that can apply to people who aren’t exactly “friends.”
All this to say that, without additional context, the word friend by itself isn’t enough to tell us who you’re talking about in an example like yours (Which friend?), so the name (John Smith) is essential (or restrictive) and shouldn’t be set off by commas.
By contrast, people generally have only one spouse at a time, making commas the default when a name follows a word like husband or wife: “She wanted to see her husband, John Smith, before going out.” In that sentence, the word husband alone narrows things down to one person; the name gives extra information that isn’t essential (it’s nonrestrictive) and is thus set off by commas. (They’re called spousal commas and not friendship commas for a reason.)
For more on commas like these (and why even spousal commas can be omitted sometimes), see “Your Dog[,] Smurf: Understanding Commas with Appositives,” at CMOS Shop Talk.