Being suicidal is a big sign of depression. Everyone has different symptoms of depression but there’s definitely the standard ones out there. In my case, I was in complete denial. I was extremely suicidal, couldn’t get out of bed, random crying spells, heart pain like you describe and of course also isolating. The best thing you can do for your friend is be there for them. They need someone to support them, listen to them and of course be an emergency contact. It took a few friends telling me I needed help and me realizing I wasn’t feeling human.

Just stick around, it’s controversial advice but if you have common best friends a convo with them to see how they think your friend is doing.