I have been in your friend's position and didn't respond to a doorbell or ringing phone for months.
I didn't want to leave the house really, but had to go to work and the supermarket.
I didn't want to talk about it or answer any questions.
What I really really appreciated were the people who kept messaging, and showing me that they cared and would be there when I was ready.
Lots of people messaged initially, but got bored or offended when I didn't reply.
Some people hung on long enough to meet up with me once, to hear the gossip, and then virtually disappeared because my life no longer fitted in with theirs.
But the people who kept checking in, sending little supportive messages, updating me on their own lives, really kept me going.
A few times I came home to things on the doorstep - flowers, a homemade cake, even a pizza!
To know that people care about you when the person you thought cared most of all has let you down, that's wonderful.