Found DP's daughter still vaping - do I tell her mum?

[NotAParentButDPis]

Just that really. Gave DP's 20 year-old (moved out a year ago, fiercely independent but has history of self harming) a lift last night and found a vape in the footwell of my car today. They have stated categorically that they have stopped vaping following historical discovery of tobacco, Rizlas and admittance of spliffs.
Please don't have a go at me, it's this that is the crux: I'm not a parent - would you want to know?

Of course not, she's a grown up!

No, she's 20.

Is this a wind up? She’s 20 years old!!

Nothing for you to tell, she’s 20- an adult. It’s no one’s business.

No she is 20.

She's 20 not 12.

She's 20. Just leave her, she's an adult.

Thanks all, your immediate reactions have shown I'm over-thinking this. Really appreciate your input and going to delete just to avoid being piled on. Entering into a relationship with a parent when you're not one is tricky, I've negotiatied this far but needed help with this one

I don't like my ds (19) vaping and he knows my concerns but he works, pays for it with his own money and doesn't vape in my house. No further discussion needed.

She's too old to be parented by you OP. Fastest way to break down a relationship with an adult is to snitch on them for little things.

I completely understand, thank you for your input, and to all those who have replied. I can't delete my post, but would ask anyone reading this to know that I have the answer loud and clear, many thanks