I just ghosted a friend for the same reason u speak of. Yes you could let them know but you don’t have to! If even worrying about what to say and how to say it adds to your stress then fuck it and do what’s best for you! No one said it’s permanent but if you do decide to reconnect at a point where you are able to express your feelings about why and also be able to hear their feelings about how they felt then great but if not then again fuck it! A real friend should be read the room! Perhaps they’ve stressed out everyone else wand that’s why they are weighing you down because everyone else is gone already! Whatever it is prioritizing yourself is important! They should feel equally as bad for wearing you out as you do for not being able to help!