It's a little strange that I have seen this thread now.
I don't have personal experience of living with an addict, but not far from my home a man has stood on the corner of two busy streets to beg for money. For years. I see him every single day as its on my route. Then he disappears for a while and then is back. Always a bit thinner and looking a bit worse for wear. A fair haired light skinned man, standing in our blazing sun (live in a hot country) and his skin looking more weathered as the months go by. People got to know him over the years, so so many people offered him a job, we all know he is a fully qualified mechanic, but at each offer he told everyone to get lost as he was able to get more money by begging. I hadnt seen him for a while over the chrismas period but saw him again last week. Again he asked me for money, I said no sorry but would he like a packet of crisps thst I had in the car. No he says only wants money. My reply as usual was sorry, and once again told me to get fucked.
Havent seen him for a few days and just this afternoon I received a whatsapp notification from our area councillor who had been trying to help him for years, to let us all know he was found dead this morning under a tree near his begging spot. He was 32. They go on to mention that he received honours at school, was a church going chap pre addiction. They also say what a bright man he was with the most amazing sense of humour, astute with inner spirit, but only when he was high. When he wasn't he was telling us all to get fucked with our job offers and packets of crisps.
I know this may not mean anything to you at all, as you say your partner is losing friends on a monthly basis. But this man was 32 years old.
How is your partner different from David? This man chose heroine over food. I watched him waste away over the years. He told me to go fuck myself for years because I didn't give him cash to buy his fix.
He didn't want any help. People tried over and over to help him. His family disowned him. All he wanted and needed was that fix.
Imagine finding someone you have tried to help for years and years to no avail, laying dead under his tree one random January morning.
Your post just came at a time when I'm feeling really quite fucking sad. This could be you one morning OP, finding someone you have put your all into for a long time, to no avail. And I'm afraid that will rip you to shreds.