You really don't know what the official reason for your termination is. What your relative surmises is hearsay and not admissible in court. Apply for unemployment. If your employer denies your claim and you appeal, they have to give a reason for termination. At that point, if they use the drug use as a reason, you can contact an attorney about your chances at winning a lawsuit. You can also request a look see at your employment file. You may copy anything in there, though you cannot change anything that is in there, nor can you take anything that is in there. Beyond that, they can usually fire you for anything or nothing. You essentially do not have a job and you aren't going to get your old one back. don't wait to start looking for a new job.
Look up "At will Employment" You can be fired for any reason or no reason You have no claim
They need proof. What have you been doing online?