Let's offroad / mountain bike / mtb / ride dirt

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.

Thanks again for the input!