Hi Gareth,
Yes, your friend can reapply for ESA as long as he can meet the contribution or financial criteria.
If he can prove that his condition has worsened since the decision, or if he can show that he has a new one/new ones, he should be paid the assessment rate. If he cannot demonstrate this, his claim should be accepted but no payment made until/unless he is awarded ESA at assessment.
Nothing on this board constitutes legal advice - always consult a professional about specific problems