Unfortunately, there are illnesses that push you beyond sanity or ability, without knocking you into unconsciousness. Those illnesses are sometimes associated with prejudice and bias, prohibiting the sufferer into not talking about them. (*)
Possibly, the employee in question suffers such a condition? How to approach him/her?
Yes, the employee must somehow notify the company.
Agree on Mail
Self explaining. Agree that a mail is send, or even a personal message in chat, and that this mail/message reaches every person that needs to know. That message possibly should be prepared, so that all that it takes is to press the Send-button.
Instruct to ask someone else to notify the company
If he/she is too disabled, but lives together with someone, that someone should be instructed to notify the company in her/his name.
Dead Man Trigger
As a last resort instruct her/him to set up a dead man trigger. This could be as easy as an email scheduled to be send in the morning. If the person is not disabled in the morning, he/she would just cancel the sending of the message.
If there's enough intimacy and trust, the person may open up to his/her colleageus and tell them about the situation, or about the fact that she/he is sometimes simply disabled because of some severe condition. The team may agree that this person can schedule the sickness-message to a point of the day that is more acceptable to her/him. If this is applicable depends, of course.
To give just two example for disabling conditions that don't knock you out, I include this section. The world is really not as trivial as the lucky sometimes think.
Migraineurs are often confronted with ignorance of the "Everyone has headaches sometimes". But that is not migraine. Migraineurs have the Sometimes-Headaches, too. But additionally, they suffer migraine episodes which push them towards the boundary of sanity. They may have distracting, visual appearances (Aura). Light, noise, light touches can become extremely painful. The ears can become extremely sensible, to the point that you cringe from very silent sounds which you otherwise don't recognize at all.
Smells, certain food ingredients or something else may trigger migraine. The possibilities are endless, likewise the variations of the pain, which several times more severe than your I-drank-two-bottles-of-Vodka-and-Broke-the-Table-with-my-Head-headache. Even if you have painless migraines, you might be absolutely uncomfortable with the situation, to the point that all you can do is go find a silent and dark room.
Migraineurs are often confronted with incomprehension or underestimation of the "well, I have headache sometimes, too"-kind, which is even worse, so they often hide that illness from you.
Explaining migraines to the non-migraineur is not trivial. Cluster headache is. And the attacks are not pushing the sufferer to the limit of sanity, but cut the sufferer into pieces throwing them over that limit.
When you suffer cluster headache, it may feel as if a glowing hot knife is stabbed through one of your eyes, from the inside. Your left half of the face may feel like in a heated iron mask with nails on the inside, and someones rubbing it forth and back. Possibly, it also feels like you're on one of those torture machines that slowly drill a screw into your neck. Women who gave birth report that cluster headache often is worse; consider giving multiple births a day, and you get a picture of the severity that some clusterheads suffer.
It often takes many years before they even know that what they have has a name. Many doctor's don't know it, and will send you through CT, Röntgen, get a checkup on skull cancer, and so on. The whole process may (will) take years 'til a diagnose.
Cluster headache is rather unknown in the public, it's so surreal that clusterheads do not expect any "civilian" to understand the pain, and so they do not want to do that. Just have a look at the YouTube videos on cluster attacks, it looks sick, it is, and one does not want to talk about those moments you whinge like a baby.
Cluster attacks can last several hours, many times day (for episoders more commonly at night), and exhaust you to total inability. There is no cure, and it can take months or years till a medication is found, if at all. Also, even if the patient is having the acute attacks at night, it may cause so much exhaustion that the sleep that follows is so deep that only physical interruption wakes the person up.
Of course, not every cluster attack is that severe that you're feeling like quitting this all. Some persons regularly have that degree of pain, some fortunately not.