Long time old friend has had intermittent mental Heath issues since teenage years (depression).
She has been uncharacteristically quiet and reclusive for the past month and has missed/is about to miss some social occasions that she'd usually be front and centre for.
All she has said (breaking a non-responding silence on phone, email, WhatsApp and social, which she is usually all over) is that she wants to be left alone to work, see her family (as in her parents and sisters families, who she's extremely close to; she's single and hasn't had a long term exclusive relationship).
I am guessing at a breakdown but she has said she "doesn't want to go into" what's wrong, so I don't feel like I can ask.
What does a very bemused good friend do in this situation? Her actions are impacting other friends who feel at best ignored and at worst aggravated. She owes people money (not huge amounts, under £100 to a couple of people) and hasn't RSVPd or booked accommodation to a wedding 4 weeks from now.
How can I help her or should I be leaving well alone as requested?
I'm not sure what you can do. Just make her aware your door is open and you are happy to listen. As she has other family around she is hopefully receiving some support. The debt is probably the last thing she needs to be worried about or perhaps the reason she is stressed.
Send her a message saying you understand, that you'll wait for her to make contact again and that you love her/will always be there when she needs her. And then yes, back off for a bit (though personally I would send a card or a message if it's her birthday before you see her again)