Why do friendships have to be so rocky?

My friend has been dating this guy for 3 yrs. she's got a 1yr old by him and he's got 2 other children from a previous marriage. He was a drug/alcahol addict. He has been clean for 6 years. They recently let a friend move into there house until she gets back on her feet. She smokes pot and has recently been smoking it with my friends boyfriend. He had a few drinks last week but told me that it made him feel ill and would never do it again. Now he is onto a new drug called Salvia which is apparently legal in most states. I'm really worried about him and weather or not he could fall back into a harder habit to break. My friend suspects that her roomy is smoking pot but is not sure. I want to tell her the truth but am scared that it will cause a lot of drama. I've known about her roomy smoking pot for a couple months now and just barely found out about her boyfriend. Do I tell her and lose 2 friends or do I just keep my mouth shut and possibly lose her friendship once she finds out I know everything? And do I wait until after the holidays? My friend will most definitly kick her roomy out and her roomy has no where to go till summer. What do I do? Should I call the b/f and the roomy and tell them to tell her or I will do it?