I'm really sorry to hear it.

There's already good advice in this thread and I don't have anything to add about what to do regarding options for wellness checks.

I think it's important not to frantically contact them over and over or to make demands about them needing to contact you (not implying that's what you would do) but I would try to contact them in a personal way if possible and write them something heartfelt about how you value them as a person and explaining how the world is a better place with them in it.

I'd move on to the next phase where I would empathise with their struggles and try to recognise their strengths and their resilience where possible, but only in a genuine way. (As a person who deals with chronic suicidality I hate when people give me throwaway lines when I am in a particularly bad place where people say something like "You are strong and resilient, you'll get through this!" and I am like, MFer what do you mean "through"?? I am this. There is no through. Can you please stop compounding my feelings of isolation and helplessness?)

I would sign off by assuring them that you understand their need to take time to prioritise themselves, that you support them doing that, and that you will be here to pick things up where they have left off whenever they are in a place to reach out again and that if they feel like they need your support but they can't sorta "do" the normal being-a-friend stuff because it's too much right now that they are always welcome to reach out for support at any time and that if they tell you that it's urgent that you'll do everything you can to prioritise it asap.

I wouldn't say that you will immediately drop everything to provide support because that's both an unrealistic expectation to set for yourself (what do you do if you're not allowed to use your phone at work or you're asleep and you didn't hear your phone go off?) but also because a lot of people say "I'm here for you" or "reach out any time" and a person who is suicidal will try to cash in on those assurances when it's things are getting really bad only to find out that "any time" actually means "during waking hours but also don't call me when I'm at work because I don't want to deal with it then" etc. so yeah... it's also important to set realistic expectations for them because there's a good chance that unrealistic expectations have failed them in the past so it's reasonable to assume that they probably feel jaded when someone tells them this stuff and it's important to recognise that and to (possibly) validate that experience.

Obviously avoid any guilt, shame, blame or demands.

I would personally avoid discussions about reminiscences except where they have done things that have made a significant positive impact on your life personally because talking about the impacts on other people or bringing up the better times can highlight the contrast between those times and how they are feeling in the present. It can bring up feeling of loss and grief but also it's important to understand that while those times might have been good in the moment, there may have been traumatic events associated with that time, including ones that you aren't aware of, and an acutely suicidal person can kinda latch on to the narrative that even the good times are terrible or it can make them feel even more isolated ("I'm glad you had a wonderful time back then but you have no idea how horrible things were for me at home. I guess your enjoyment of life takes precedence over my experience of suffering..." kinda stuff.)

Continue living your life and know that ultimately suicide is a choice that a person makes, generally because their experience of suffering is completely intolerable to them at that point in their lives, and that unfortunately if someone is truly determined to take their own life then they will find a way to do it; suicides can happen under strict surveillance in psych wards with a team of trained professionals whose entire job is to make sure that doesn't happen, and it's not like you have anywhere near those resources. Sucks to say it but that's the reality of it.

Try not to take on too much blame or responsibility, just do what you can even if you are extremely limited in what you are able to do for the other person, and hold hope for that being enough and for things improving for them.

I hope that things are okay, that you are taking care of yourself through this, and that the feelings of guilt aren't too persistent.

If you want to pick my brain about my experience of suicidality to get a better understanding of it/how to handle suicidality or to support someone experiencing it, go for it - I can only speak to my personal experience of it but also there's not much that is off the table for discussion either and you'd really have to try before you'd strike a nerve.