The problem with really cheap bikes it you're basically buying a toy unless you spend a bit more, you certainly don't need to spend a fortune, but £300 for a hard tail MTB is really pushing it.
"Toy" is probably the correct term, despite it potentially sounding snobbish/elitist.
A cheap bike will have minimal/no consideration in the design and build with regards to strength - so will either be heavy or not really suitable for long-term mtb use. (more like a hybrid and a grass field or canal path is as "off road" as it gets). Some cheap bikes are cheap steel (but not like a refined Ragley steel mtb) so will be heavy and prone to rust. Also cheap bikes will have awful components - HEAVY forks, rubbish brakes and a drive train that wears quickly and needs regular tweaking - I've seen all this in the past.
All in all, it makes for a less enjoyable experience. I went through this with my gf - 10+ years ago we both thought we'd get a cheap-ish mtb from Halfords and go out together. I did a bit of research and went for a Voodoo Bantu for £300ish but she didn't want to spend so much and got a £150 job. Her bike was SOOO much heavier than mine, a lot less comfortable and just not happy on anything away from a canal path, not to mention weighing a ton when you wanted to lift it in/out of the car, around turnstyles and just sluggish to peddle, not to mention it kept playing up with the cheap parts. Within 6 months I found a sport I loved and she was sick of it...
Today, it almost seems that £450-500 is now the cross over point for a "toy" vs a bottom-range hardtail. I would 100% suggest it's worth spending that little bit more to give her the best chance of enjoyment out of it, especially if you already have a semi-ok bike and you want to stick together.