The question of whether to live on is ALWAYS whether the rest of your life is going to have enough pleasure to make it worthwhile. That's what Stoics don't want to admit and that is a reason this question frequently occurs. It is certainly possible to consider circumstances in which you love a friend so much, and you can see that (for example) if you betray that friend to torture by an enemy when you could easily avoid it, that you would be so tormented by the pain of that decision for the rest of your life that you would judge continuing to live would not be worth it. That is, in such a situation you might calculate that the pain of continuing to live would outweigh any remaining pleasure, and you would choose to die trying to save the friend rather than walk away.
DeWitt makes a lot of sense that pleasure has meaning only to the living, and so that all discussions of pleasure or pain presume that one first has life. I think the difficulty is transferring that observation over into the context of discussion a “Greatest goodf” and that is probably a stoic or platonic corruption, as Epicurus implied in the quote from Plutarch about walking around uselessly debating the meaning of “Good.”
If one wants to debate whether DeWitt should have said “pleasurable living” rather than “life” is the “greatest good,” or whether “greatest good” is a legitimate concept in the Epicurean framework, or whether “good” has a special meaning other than “guide” - then fine. But the objection that dying for a friend would necessarily make one a Stoic , or contradicts Epicurean ethics, is totally unfounded.