How to confront a situationship over their drug use

Okay, this is a wierd situation. Context: I’ve being in a situation ship on and off for about 7/8 months and we’ve grown very close, to the level I met their parents& our friends groups combined. , we have been off and on for a bit, mainly due to issues stemming from their drug use. When we met they were sober, and relapsed months later on cocaine when their grandmother died. They recently relapsed a month ago, and were in a better headspace shortly after and wanted to stay clean. they have been getting more verbally aggressive, drinking excessively, and becoming very hostile. To the point that we recently stopped speaking for the moments I’m very aware due to their nature that they’ll come back and apologize, like after most drug induced outbursts.

I found out today that in the last week where their behavior was more erratic, they had been using again. Via a third party, so they don’t know that I know. We aren’t currently speaking but it’s a cycle when they act out they return.

how do I tell them I know, how should I go about this, and what is the healthiest plan of action? I do know their parents so I could reach out to let them know to get them help. But I’m honestly out of my depth here. I care very much about them, and I’ve seen this behavior Before and they’re on a much more prolonged bender. I’m worried for their Health and safety and want to encourage them to potentially seek help without triggering combative defensive behavior from them. What would you do?