Sorry to hear about your friend and how the relationship has changed, it's never fun that's for sure. I've had numerous friends/relationships that went this way, a few times I was even playing the role of your friend (only difference is K was never the drug, usually heroin or E).
*This is just my opinion or experience by no means am I telling you what to do, you seem like a smart cat*
If they are a close friend, someone you would confide in, go into laughing fits with etc, then I would prepare yourself and get them alone with you. If you think the K is damaging his life, or will end up that way then let him know. Have a serious conversation, regardless of how uncomfortable it may be. Let them know how you feel. But importantly focus on why they are going this route. Do they have a void they are trying to fill, are they depressed, or feel alone? Again, every situation is different and I'm sure others will have differing opinions.
If he is isnt a super close friend, but was still a good buddy, I would still let him know how you feel.
Example: I had/have a buddy who I considered my best friend for a solid 4 years. We went to high school together, college together, and after college lived in the same city (we didn't live together though.) He started getting into drugs casually during college. Slowly it grew from casual to abuse or addiction. It wasn't any particular drug, he just wanted to get high all the time, on anything he could. Hell he once made crack in his family's kitchen...... eventually it became a heroin addiction. I would do drugs with him sometimes, but once I felt I was enabling his behavior I stopped doing drugs while hanging out with him, but he still did them. He would beg for rides to his dealers house, ask for money, or claim I owed him money when I didn't because he needed to cop a sack. It got bad.
The worst part was that when he was sober, he was the coolest fucking guy. But when he was high, he became a total asshole, another person. Anyways, long story short, after rehab multiple times, and relapse every time, his other good friend and I basically said either stop, or consider our friendship over. To our face, he chose the drugs.
It pains me daily. We tried to help more times than I can count in so many different ways.
It can be a personal thing for you Joseph. Do you enjoy your time with him still? I would try my best to help him out. But life is never fair, and he is his own person. Better not to let yourself go mad, or get hurt, because of someone else. I wish I could provide better help or advice.
But you aren't alone. I've lost plenty of friends, girlfriends and even had a falling out with a sibling (luckily I repaired that one) to shit like that. At the very least, tell them exactly how you feel, and offer them help.
I wish you the best, just know that even if it doesn't work out for the best, I'll bet you have plenty to still be happy and thankful for.
