Likewise I have kept every text, email, card or letter that I received after my son died. It proves to me that his life mattered to other people as well as me.
Keep in touch. Don't not ring her. Silence is dreadful and it can be very hard to pick up the phone. Your friend may not pick up but will listen to any messages you leave.
Offer specific help - intimes of distress it's impossible to think what you might have help with.
Cook a freezable meal, offer to go to florist/dry cleaner/shop etc.
My best friend went out and bought me 4 dresses to try on for his funeral. I simply could not go in to a shop and look for a dress to wear at my own son's funeral. This was probably the most intimate, most helpful thing ever.
Keep in touch. Say her son's name.
Remember birthdays, anniversaries. Dates that are hard - Mother's Day etc. Say his name.
Keep in touch.
Do not be afraid of her grief because her grief is way worse than your fear of seeing hers grief. Cry with her.
Do not say there is a reason for this. Do not say he's in a better place. Hold her and agree with her that this is shit. Do not try to make her feel 'better' as that feels as if you are minimising her pain. Her pain is her love and her love is her strength.
Keep in touch.
Keep in touch. Even when she does not answer nor ring youback.
Keep in touch.