Hi @Till23
I have a few suggestions based on what helped me when I was experiencing a very similar feeling for a period of time not so long ago.
I started out my just trying to do small things, things that didn't take very long at all, maybe even just a few minutes. It was almost impossible for me to feel any enjoyment so by starting with these small things, it allowed me to begin to build a bit of a habit without putting pressure on myself to 'have fun'. I also started trying things i hadn't done before. Since i wasn't enjoying anything, doing something new gave me a bit of interest that just wasn't there for things I had enjoyed before.
Even just forcing myself to get off the couch or bed for a short time, just to get moving felt like it helped me to do other small things. In the beginning all of these small things were things I had to make myself do, but slowly it started to become easier and I found i could do them for longer without feeling like it was an imposition.
It also helped to sit somewhere soothing. For me it was outside looking at the ducks that would use the pool. I figured it i was going to sit somewhere and stare into space... it might as well be somewhere nice and relaxing, and somewhere that made me feel some little happiness.
For me, i knew the major cause was depression about some major stuff going on in my life, so i started to reach out out and try to find a little bit of support in dealing with that. It also got me more involved in supporting others which helped give me a sense of purpose which also helped a lot.
Throughout all of this it was important to remind myself that I wasn't doing it to feel happy or like I was enjoying something. If i thought like that, then I knew I would give up because it wasn't working. The goal was just to get myself moving and doing things. In the hope that eventually that enjoyment would return. I needed to remove all pressure on myself at the start so i didn't feel like i was failing. That's not what it was about at the start... it was just about doing anything, even if for brief periods.
It takes a while to work through, but eventually i started to find i started to feel some enjoyment doing things again and i could do them for longer and longer. The key thing though was to work on the root cause and tackle that at the same time.
Any yes, I did feel like i was being lazy, which was why I didn't talk about it. It felt like that's all anyone would tell me.. "get off your butt and do something!" It wasn't even a motivation issue. It wasn't a lack of motivation, doing anything just felt pointless. Life felt pointless. I had no wish to end it, but it just felt like why bother doing anything.
So maybe talk to this person about what they feel is behind this feeling for them. Remind them that anything they can do to get out of it, even something tiny like getting out of bed is a big win. I hope this person finds a way though soon. It's a horrible place to be in.