Please try not to diagnose people you've never met or spoken to on the internet with diseases. You can suggest they may have something, and recommend they see a doctor, but you seem very sure of your diagnosis.

It may be a depression. But an existential crisis is possible without a depression. We change over time. Our ideas evolve. At some point, what we have been doing may seem pointless. Time to take another path to find purpose, meaning and/or love again.

Consulting a psychiatrist to ensure it's not a depression is a good idea though. If not, some psychologists are specialized in existential crisis ; it's called "logotherapy".

Could be, but not necessarily.

It could also be the result of a logical and intelligent thought process. I think anyone that follows the data and truth will eventually come to an existential break down that reframes their mind. Especially if they were raised religious, but that facade starts to crack.