Thanks for sharing @LexiNico - lapses are common with any kind of behavior change, still they can be concerning, as you are highlighting 
They can be less concerning when the conditions are safe, they last for a short time and they are used as a learning experiencing (what conditions lead up to the lapse and what was working in recovery to get that sober time?) Even less concerning when they have a professional partnering with them through this learning process.
All that to say, you have every right to feel concerned if when you realistically evaluate the situation you feel it is less of a blip on the radar and more taking hold of them back down the using path.
Thinking through this with you there are a couple ideas that come to mind:
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Slow and steady: Since it sounds like it is hard to tackle a conversation head on about the lapse, maybe working on getting connection and the communication lines back open through positive communication tips - eg. what other harmful behaviors are occurring that can you agree on? This helps you begin to work on positive actions instead of just banging heads about the usage. Also, try asking for permission to talk about the lapse - eg. "I love you, and I’m concerned about your health and happiness since you lapsed, when is a good time that we can have a talk about it so we can figure out how to move forward (with recovery) together?
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Accelerating help: what has worked for him in recovery before? How can we re-engage in those activities? Have there been any professionals that have been useful for him that he listens to (therapist, counselor, recovery coach, friend, parent, sponsor)? eg. “You seem tired of this cycle, do you think we can re-engage some of the practices / help that was working for you before, what do you think might help you out of this funk? How can I help?”
Regarding trust, when someone is in active addiction the drug use is their number one survival concern. Knowing this, we can understand that they’ll protect that at any cost. Rebuilding and refocusing trust and connection outside the drug use can be helpful to establish where trust does exist.
I do hope you’ll join us tonight since our digital meetup topic is relapse and we will be able to go more into depth on and talk through it, I think it will be very useful for you (register HERE.)