Thanks for all the informative answers! Should have specified a budget obviously, sorry. It's the same as always: not too much please ;) So to summarise
a well fitting frame and decent wheels. Ok, no different from a road bike.
a hardtail is enough to begin with.
2nd hand 26" is a good low budget option
but not one with a crap fork
She's 167 cm btw. And easily scared of speed. I've no idea if she'll be into this or not once she's tried it out.
Truth. 26" kit used to be bargainous but now it's getting rare it's just not the case any more. Unless you have a good reason to go 26", stick with 650b, there's so much more out there. Bit sad, really.
Thanks for all the informative answers! Should have specified a budget obviously, sorry. It's the same as always: not too much please ;) So to summarise
She's 167 cm btw. And easily scared of speed. I've no idea if she'll be into this or not once she's tried it out.
Thanks again for the input!