New MTB advice, please.

With your proposed budget, it would be really annoying to end up with a bike that was a poor fit. You could try looking into geometries and reach and stack, but getting consistent information to compare bikes between manufacturers is tricky. The best thing is to make a short list and try for her to ride (at best) or at least sit on as many as she can. I once bought a bike online (a Canyon) that on paper looked to be a sure thing. I unpacked and assembled the bike and within 10 seconds of throwing my leg over it for the first time (in the kitchen!), I knew it was not the right bike for me, so back it went. Half a dozen test rides later, I found my Camber and we've been together ever since

Being a "very serious roadie" will probably mean that your daughter has a full carbon road racer, one that is extremely light by mtb standards. And of course she will be seriously fit and also skilled at road biking. MTB is different. The bikes are heavier because they need to be, but lightness comes at a cost that is not always worth the money spent. The money should be going into the suspension.

MTB skills are different! Your daughter may object, but I advise her to take a couple of MTB skills courses. I would start off on an introductory skills course. The word "introductory may put her off (a lot), but you can't beat getting the basics right and at least it will establish the need for more. Depending upon how that goes, move on to an Intermediate skills course. That might make her think!
By "skills" we are not talking about teaching someone how to ride a bike, but how to ride an mtb on a rough and unstable surface. We are not talking wet cobbles here or smooth tarmac where you can do 70 mph. There will be rocks, drops, wet roots, flat or off camber cornering on rough terrain, traverses on loose or slippy surfaces, steep climbs on pebbles, descents on loose surfaces that mean you are going down whether you brake or not!

The partner of my elder daughter is a triathlon fanatic, has raced for his country etc. He is super fit, muscles in his spit etc. He has 8 bikes ranging from a steel tandem to a high tech carbon road racer that weighs 3/4 of naff all, and everything in between. Amongst his stable is a 100 mm travel full suspension mtb (carbon of course and dripping with top tech). Occasionally I run MTB guided rides and he came along to one of them. There was us; of varying fitness standards (dodgy mostly) in our grungy gear and with our heavy mtbs (by roadie standards). Most at least 20 years older. And there was him looking like a tanned Greek God in his cling film lycra astride his ultralight XC racer. Bloody Hell, we almost gave up and went home! On the smooth initial stuff out of the car park, he left us for dead, but as soon as it got a bit technical, he was in trouble. He didn't actually come off, but there is no doubt he lacked the skills to tackle what we were about. He has not been back!

If your daughter has never done mtb before, she may hate it, she may love it, who knows? My advice would not be to spend £2500 (serious money for a hardtail), but to get a good used one for considerably less money and then go on a few courses on trail centres near you. Hmm, just remembered we are in lockdown so there will be no courses available. OK, she can have a go at the mtb trail centres, preferably with a mate who is an experienced mtb rider (not someone who like her is giving it a go).. Start on a blue and if that goes OK, try the red. But for heavens sake council her to take it steady to start with. The problem is that she may start to develop bad habits that will be difficult to break once she takes some lessons.

Best of luck! Please let us know how she gets on.

One last anecdote.
My 8-year old grandson was tackling a double rock drop of about 12 ft for the first time. I had been preparing him for quite a while on smaller drops. As he readied himself for his first attempt, three women in their mid to late 20's on their 29er Xc hardtails turned up to watch. He aced it on his first attempt, made it look easy in fact. So easy that one of the women declared that she was going to have a go as "even a kid can do it"! I cautioned her, but she was adamant. I gave her my best advice and managed to limit it to three easy to remember points. Off she went and came a cropper. As her mates were scraping her off the trail amid blood snot and tears, my grandson went sailing past on his 24" wheeled rigid bike, even faster!

There are skills to be learned. 'nuff said!