<p>I would just say “During that period of time, I was on a leave of absence from college because of a medical problem.” If the student was able to accomplish anything (e.g., held a part-time job, took an online course, did volunteer work, cared for younger siblings) during that period, it might also be a good idea to add “Although my medical condition precluded full-time study at that time, I did [hold a part-time job in a retail store – or whatever it was] during [three months – or whatever the length was] of that period.”</p>
<p>I wouldn’t suggest lying. If the student indeed took a medical (rather than personal) leave of absence, there are records that show it.</p>