The difficulty of writing about a dead friend is not finding the words. The difficulty is finding the right kind of honesty — the one that does justice to the person rather than to your grief, which is a different thing.
Grief wants to simplify. It rounds the person into their best qualities, files away the friction, makes them good in a way they were not always good while alive. This is understandable. Grief is doing its work. But writing that serves only grief produces a memorial — a representation of what the person meant to you — not a portrait of who the person was. And the person, if they could read it, would probably recognize the difference.
The harder question is what you owe them. You owe them accuracy, which means including the parts that complicated them. The friend who was sometimes unreliable. The one who was generous with everyone except in one specific way. The one whose humor had an edge that occasionally drew blood and who knew it and didn't always pull back. These are the features that made them a full person, and writing that omits them makes them smaller, not larger. The dead deserve to be complicated. Smoothing them out is a quiet act of diminishment disguised as reverence.
But you also owe them care with what is private. There is a difference between truths that illuminate a person and truths that expose them. Some things they told you in confidence, not for the record. Some experiences belong to them and to you and not to whoever is going to read what you write. The ethical question of what to include is not answered by the rule "only good things" — that rule produces flatness. Nor is it answered by the rule "tell everything" — that rule produces a violation. The answer is closer to: what would a fair witness say? What does this reveal about who they were, and does the revealing serve the truth of them or only my need to process mine?
You are also writing yourself into the story, whether you want to or not. Writing about a dead friend is always partly writing about who you were in relation to them — what you saw in them, what you brought out in each other, how they changed you, where you failed them or they failed you. This self-implication is not vanity. It is accuracy. A friendship exists between two people; a portrait of one that leaves the other out is already incomplete.
The specific problem of writing after loss is that you cannot revise it with them. Every other form of writing is correctable — the subject can read it, object, add context, correct the record. The dead cannot. This gives the writer an unusual power and an unusual obligation. You can make the person whatever you want. No one who knew them as well as you did may contradict you. This is a kind of authority, and authority requires restraint.
What writing about a dead friend can do that no other form of attention can: it can hold the full person in view at once — past and present, complicated and tender, as they were and as they are now in your understanding — and try to get that combination true. It can make a record that their children, or yours, or strangers who never knew them might read and think: I understand who this person was. Not as legend. As human. That is not a small thing to offer the dead.