If you searched for this title, I want to say one thing before anything else: you are living the single most common situation I encountered in years of cofounder litigation. Not fraud, not betrayal, not stolen code. Just this: someone who was all-in has quietly stopped, is not really gone, and still holds their claim on the company. You are not overreacting by taking it seriously. The teams that ended up in my office were mostly teams that waited.
Here is how to think about it clearly, in order.
First, diagnose before you act
"Stopped working" covers three different situations, and they call for different responses.
The rough patch. Burnout, a family crisis, a health issue. Temporary, acknowledged, with a horizon. This is what partnership is for; the response is support, plus, at most, a written note of what you both expect over the coming months.
The silent drift. Fewer hours, slower replies, energy visibly elsewhere, no conversation about it. The most dangerous case, because every week of silence hardens both positions: yours ("he is not pulling his weight") and theirs ("I never said I was leaving").
The de facto departure. They have a new job, a new project, or simply vanished, and everyone knows it except the paperwork.
Be honest about which one you are in. The rest of this note is about the second and third.
The conversation you are avoiding is the whole solution
Every file I handled contained the same artifact: months, sometimes years, between the moment one founder knew and the moment anyone said it out loud. In that gap, resentment compounded, evidence accumulated selectively, and positions hardened into legal postures.
So the first option is always the same, and it is not a legal one: a direct conversation, this week. Not an ambush; an honest reckoning. "Here is what I observe. Here is what the company needs. What is true for you right now?" Three outcomes are possible, and all three are better than silence: they recommit (with expectations now explicit), they admit they are done (and you move to an organized exit), or they refuse the conversation itself, which is an answer too, and one worth having on record.
Whatever is said: write it down afterward, in a message to them. "As we discussed today..." Not as a trap; as hygiene. Memory is a terrible contract, and every file I litigated was, at bottom, two honest people with two different memories.
Your options, depending on what exists in writing
If you signed a founder agreement with vesting and leaver provisions: you are in the situation the paperwork was built for. Their earned portion is what vested; the unearned portion returns under the schedule; the manner of exit follows the leaver definitions. The conversation above still comes first, but it happens with a script instead of a void, which is precisely why the outcomes are so different.
If nothing is written and you are pre-incorporation: better news than you think. There are no issued shares yet, only promises. This is the cheapest moment there will ever be to formalize reality: a founder agreement signed now, with vesting backdated to the real start and each founder's situation stated honestly, converts an ambiguous drift into a documented deal. A disengaged cofounder will often sign a fair agreement precisely because it recognizes what they did contribute. What they will fight, later, is a company that succeeded and now claims they were never really part of it.
If nothing is written and you are incorporated: the equity exists, and unwinding it requires their agreement. Your tools are the negotiated buyback (fair, fast, documented) and, failing that, professional advice, because the alternative paths depend heavily on your jurisdiction and facts. What you should not do is what most teams do: nothing, while the company grows and their leverage grows with it.
The clock matters more than the conflict
The single variable that most determined outcomes in my files was not who was right. It was how early the situation was addressed. Early, everything is a conversation between people who still respect each other, and the numbers are small. Late, it is a negotiation with a stranger holding an asset. A ghost cofounder is not a people problem; it is an unwritten-consequences problem, and consequences can be written at any time before the leverage arrives.