Help Advice Needed on 18 yr old Child

Sorry this will be long... Bit of a back story.

I'm at a loss. I'm a mum of 3. They are all over the age of 12 with my oldest being 18. We have been through a lot due to my ex husband who was emotionally and verbally abusive to me with the occasional physical abuse towards the end.
My 18 yr old finished school last yr and looked like they had a lot going for them work wise etc. Towards the end of the year they started dating a new person. At first I thought nothing of it until I started to notice that this person was never at our place (they always stayed at this persons house), when I did see them they would never look me in the eye or talk, that my child was changing in ways that weren't like them and that my child was not spending time with their other friends. Just to name a few.
I started to watch more what was happening. My child was emotional, having massive outbursts, was withdrawing from us, becoming disrespectful, having issues at work, was dropped from a management pathway at work when only months before they had big plans for them and they were depressed.
It came out that the new partner was a pot smoker and even though my child has never been for drug use due to their father, they had done it with them. I then started to notice that in the days following them smoking my child would be manic, abusive, withdrawn and just not them.
I recorded one such outburst and played it to my child a couple of days later. They were embarrassed and wanted me to turn it off. I explained that I didn't think it was effecting them in a positive way and the effect on them was not good (we do have issues in the family that they are aware of that this seemed to trigger in them).
But I noticed that when we spoke about this they kept saying; I know you don't like them and want me to break up with them.
I'd never said this. I might have wished it but hadn't said it or done anything to say it.
After this I saw messages between the two of them a couple of days later. I actually wasn't looking but went to use the computer and everything was there in front of me when I turned it on. My heart sunk. This person and the way they spoke to my child made me think of my ex. The messages I saw were them calling me the most disrespectful things because my child was going to help me with their younger siblings. I saw that if my child didn't answer this person they bombarded them with messages of guilt and abuse until they did. I saw this person spending my child's money and knowing exactly what was in their account. I saw things that made me scared for my child.
So I went looking. I have access to my kids accounts. I logged on and saw my child who had savings now had nothing. Near on $2k gone. This was savings for a trip in the middle of the year they had signed up for with another friend to go overseas.
I spoke with my child and found out in a roundabout sort of way that they were the ones paying for everything as their partner wasn't getting much work. I let my child know that if they also got caught driving whilst under the influence they could kiss their trip overseas away.
My child went away with their grandparents and had no contact with the partner for 2wks. But as soon as they got back I got a bad feeling. Within a wk my child whilst I was at work had packed a bag, wouldn't return my calls, wouldn't reply to text messages and when I finally got to speak with them got told that they were 18 and could do what they wanted I had no control over them.

This was not the child I knew, loved or raised. I reminded said child I still pay for everything including the phone in their hand (they do pay money to me for it but it's in my name as they were under 18 when we got it) and have just paid the Europe trip due to them having no savings but they were right and they are 18.
The worst thing I did was go to my child's work as I wanted to pass my details on to my child's partners parents so I could make sure they are alright and safe. I've never met them, have wanted to due to what was happening as I was told they were ok with the pot use and supported it. I found out that my child's work is worried about them, that it's because of the partner that they have been taken off the management pathway and that they have seen such a change in my child they thought they were on drugs.
I went home and cried. I saw my child from a distance whilst they were at work as I was leaving and they had marksup their neck (hickies) which they hate, they looked miserable, they looked unkept, they looked withdrawn and it wasn't the bright happy child I knew.

My question is; Do I sit back do nothing as they are 18 and it's their life and just been there when everything goes to shit like I know it will.
Or do I fight for them because I know this is not my child, the new partner is abusive (emotionally/verbally) and do everything I can to protect my child?

I know I haven't given much info but I'm lost. There's so much that has happened in such a short time that I'm honestly in shock. I'm watching my child change in front of me and it's not for the best.

I've had friends tell me it's my fault and to leave them alone. I've had others ring me over the last couple of months worried as they have been seeing the same changes and wanted to know what was going on. I've had family worried one minute then telling me to back off and leave them alone the next.

What do you do when you can see your child slipping away and changing in front of you and it's not because they are growing up and moving on, it's because they are going down a road that is having ill effects on them.