You reminded me of the green army ad xD. "Newcastle away today, that's a long old poke"
A single from Newcastle to Plymouth is £30.35 (with railcard) and £46.00 without. This is on the 9:03 from Newcastle arriving at 16:50 and a direct train! I don't think you'll get a better single to be honest (it is worth getting her a 16-25 railcard since full time students qualify irrespective of age)
Return tickets are not on sale yet since 1st May is more than 12 weeks away.
edit: all the others around it are £50 at least so try and book soon.