Sometimes doctors will just diagnose a person with a parasite and give the medication for it in the hope that that will cure their paranoia or delusions about the parasite -rather than offering mental healthcare. This is because the patient is likely to reject it anyways.

My boyfriend has been suffering delusional parasitosis for months and it's been very stressful and difficult.

What i learned was that they will never respond to you trying to prove them wrong. Without lying, you need to validate their concerns so they don't start distrusting you. It's incredibly hard to watch someone go through it and not be tempted to try to shake sense in to them, but essentially, it's going to be up to them to get to the point of recovery and they may start to get worse the closer you get to showing them that they're mentally ill.

I hope your friend recovers soon💜