Circumstances stated at re-joining smell something fishy, and the employee's sickness does not sound genuine. However, since he is a good performer, you can allow him to rejoin with written recommendations from his boss.
Communication in Today's World
It is a big surprise that in today's world of communication, where many options are available to pass on a message (telephone, mobile, fax, email, post, courier, SMS…), from any corner of the world, an employee remains absconding for fifteen days and now gives a justification that he was ill.
Responsibility of the Senior
Actually, his senior, who was aware of this sickness and did not inform HR about absenteeism, is responsible for such indiscipline. Also, it is the concerned senior's job to get the medical reports upon his joining back. When a sick employee informed his senior about his leave, the senior should have warned him that at the time of rejoining, he must bring his medical reports.
Take a recommendation in writing from the senior stating, "Employee be allowed to rejoin without medical reports, and the same will be made available to HR within a week's time." Let this senior also get involved in the issue.