The fighter could do a Partial Charge with his readied action, if he doesn't move before readying the action.
You could partial charge even if you had moved earlier, so long as it continues a straight line. However in this case the spellcaster only moved 5' away, which is too close for a partial charge. So, oddly, if you start your turn with 5 feet of empty space between you and the mage, you may want to stay put to ready; we assume in an American football type stance, waiting for the mage to go for the component pouch . . .
Should there really be a lower limit to the distance moved with charge and partial charge? Should the rule be something more like: you always suffer a -2 penalty to AC, but you only get the +2 bonus to attack when you travel 10' distance or more.