My DS was just the same.
We nagged, cajoled, calmly explained the potential consequences of messing up his GCSEs, ranted.
The school mentored him; basically we did all we could, but to the point when, sometime in late Feb I recall, I calmly sat him down and told him that I would no longer nag as it was destroying our relationship and was achieving nothing. I did say, though, that I wasn't going to go on funding failure.
I also pointed out that the villa holidays and the 4 bedroom house weren't god given rights, that our pay bought them, and that we weren't always going to be around to house him.
So from then on, every time I saw him gaming and not revising, I'd just say 'you and I both know what you should be doing right now'. I also reminded him that at no stage was he ever to attempt to blame us for his failure.
He achieved one grade lower than his predicteds in 8 out of his 10 GCSEs. Which I know he was privately disappointed by.
He went on to start 4 AS levels at a 'forgiving' 6th form; but by February, 'Maths' had been downgraded to 'Use of Maths' and Physics disappeared altogether. We had a very uncomfortable parents evening where it was made clear they were sort of washing their hands of him because he wasn't interested. This came as a bit of a shock to him as he was used to us all begging him, and dancing attendance to him. He wasn't expecting 'If you can't be bothered, nor can we, mate!'
So I acted (he had ever fewer options to fall back on himself, by this stage). We pulled him out of the 6th form (at the end of the year) and sent him to the local Tech to do an extended diploma BTEC. I told him that if he started failing that, he was going to leave and to start working fulltime at Tesco (he was very PT there). I was very calm, I presented it factually, not as a 'threat' as such, saying 'shop work in Tesco is a perfectly good job that thousands of people do happily every day of their lives. It's a pity if you end up doing that because you have to, not because you want to, but that's a choice you're making. I won't go on financing your failure'. But I know he was initially a bit embarrassed to be going to the Tech as he's a bit of a snob, but reality was dawning.
Well, finally at 17.5 he'd grown up enough to realise that he was only as good as his last set of exam results (academically, I mean!) which were D and U at AS. He knuckled down at Tech and achieved D star D star D (and the BTEC format suited him far better).
He's now in Y3 at uni doing Computing. But even now, he wryly recognises he's smarter than many of the DC on his ex-Poly course. He needed 130 UCAS points to get in but he had 160, BUT the red-bricks wanted Maths at A level. He sees this as a consequence of the eyerolling he did aged 15-17; but he was just too immature to do anything about it back then.
So my advice would be to remain supportive, offer help, but be cool and impersonal in citing the consequences of her laziness. I think it's even more difficult right now with the looming unemployment crisis due to Covid and the contracting of well-paid careers, post Brexit. There'll be a lot more competition in the market and, as others have said, sixth forms will be giving the dead certs resit opportunities, not DC who failed due to laziness.
She needs to decide which side of the have/have not fence she wants to find herself on.