I think the best thing to do would be like
@petitspois said and pull back a bit. I think it's the healthiest thing to do regardless of why the friendship has faded. If it is just that it isn't working, then pulling back your own efforts will help both of you.
But I think if it is their poor mental health, very well-minded and good intentioned people (like yourself) can accidentally put 'pressure' on that person that can mess things up a bit. If they tend to isolate themselves when their mental health is bad, it probably is a huge amount of work for them to socialise. And sometimes when a friend very kindly reaches out and tries to plan things or talk more, it can put pressure on the person struggling to not be seen as mean or ungrateful. And that pressure can make the hard task of socialising even harder, and it becomes a vicious circle. They pull back, you notice and push forward, they pull back more. I'm not sure how well I explained myself there! But it feels like this might be the situation.
I think I would simply pull back silently, play the game when they're online or want to. But put the trip planning on hold for a few weeks and see where things lie then. Sometimes the only thing you can give a situation is time.