You can still be a friend without getting into his business. Listen but do not advize, listen but do not act, listen but do not judge.
His sobriety will be reward for him and once sober for a time he will know if and when to approach his children. Only sobriety can give him the clear head to know what to do.
be a friend and "let him" get sober.
I hope I am not sounding too stern but it's what I've learned and it works for the rest of the addict world, lol.