Heroin as a recreational drug?

Well, everyone is different, but heroin is very addictive.  It is possible that your cousin only uses heroin on the weekend.  My guess is that he doesn't realize that the amount used or the length of time he spends using it has increased (maybe now it is Friday and Saturday instead of just Saturday or it is more hours spent using it).  He may also be experiencing withdrawl symptoms that he doesn't recognize.   Sometimes being addicted to something isn't obvious to the person, but may be obvious to others.  I think it is unlikely that his use of heroin is "no big deal."