You are right. I would use a second comma, as you have. That's what I'd prefer. On the other hand, if I didn't put in the second comma, I would leave out the first comma, too! "It's now not known which, if any, of the Friends stars plan to take part in the..." <---My preference "It's now not known which (if any) of the Friends stars plan to take part in the..." <---This is another way to punctuate it "It's now not known which if any of the Friends stars plan to take part in the..." <---I don't like this at all, but I think it's acceptable "It's now not known which, if any of the Friends stars plan to take part in the..." <---I would say this is actually wrong--but understandable. The concept, in any case, is: "which (if any)..." No matter how it's punctuated, "if any" is parenthetical. It's needed because "It's now not known which of the Friends stars plan to take part..." means "we know that at least one of them plans to take part." The reason for the uncertainty is that punctuation marks to some extent represent pauses in speech, and the expression "which, if any," is a stock phrase. We need it because we don't have a single word for it. So when it is spoken, it is often said quickly, as if it were a compound word, "which-if-any" or even "witchuhfenny." This makes it feel unnatural to write it with the commas, because it's not really the way you'd say it.

it's better if u don't put comma after if any