Because it's synthetic marijuana.. no offence to anyone smoking/enjoying it but to me it was total bs. Only had it once and there was absolutely nothing enjoyable about the high, if you can call it that, I was shaking, out of control and mumbling some incoherent words and I had no idea what was going on around me and I was just very scared. I think you should have a good talk with your friend. Bring him some actual weed, go out in the nature and walk around, have a jay, talk. Maybe even leave him some weed so that he can smoke it instead of the synthetic one when he feels like going to sleep for example. I'm sure he too realises that what is happening to his body isn't normal and he obviously can't be feeling good neither physically nor mentally, so just take some time to discuss it with him.