I used to go out with a cocaine addict. The problem with cocaine is that it basically messes around with the user's brain chemistry and makes it harder and harder for them to experience happiness /joy /a good time without it. This makes it an extremely hard drug to just give up.
For my ex, without cocaine, everything was boring - going out for a night on the town without cocaine was like as dull as a kids tea party. A bbq with friends was like being asked to spend the afternoon folding laundry. Nothing was fun, nothing excited him - his brain just couldn't produce the right chemicals to feel excitement or happiness or joy. I don't know how long that takes to come back.
It must feel like living in a world where everyone's idea of a good time is stamp collecting, a cup of tea and an early night.
It's a hard drug to spot in someone else - chronic users are very, very good at appearing normal, and it takes just a few seconds to rack up a quick line in a loo the rejoin the group. I knew my ex was using (cocaine use pretty normal in my circle) and even I didn't spot when it became an addiction. He was using it at times and places that shocked even me - I would be thinking "who the fuck would have a line at ten in the morning on the way to his parents house for lunch?" or "why would you rack up a few lines 'to go to bed with'? That doesn't make any sense"
The main tell that someone is using cocaine is sniffling /blowing their nose a lot. They will claim allergies. Aside from that, if you have full access to their bank account, look for large cash withdrawals at regular intervals. If they are willing, do a drugs test, but a urine check will only give you info about the last 24 hours. Aside from that, addicts always lie and minimise so assume its more than they are admitting to.
The one good thing is that cocaine is not actually physically addictive - the dependence is psychological (though that doesn't make it easier to give up) so an addict won't experience withdrawal like they would with alcohol or heroin.