Flight Instructor reported observing his student lose control of the aircraft while conducting solo landings; resulting in a runway excursion.

Narrative

My student; Person A; was conducting his third solo flight to the local area. He proceeded back to the airport; ZZZ; and successfully accomplished a landing. On the second and final landing; he began a stabilized approach to runway XXL on proper speed of approximately 70 kts. I witnessed the round out and flare from the ground which looked to be in the proper landing attitude. Upon touchdown the aircraft began to porpoise and proceeded to drift off the right side of the runway. All solo endorsements were proper completed and student document verified before flight. I have flown with this student many times in the last few weeks and observed no issues with landing technique. Prior to this flight I briefed the student on proper crosswind technique should one arise and procedures for go around and bad landing recovery. At the time of the occurance; there was a slight crosswind of only a few knots and was endorsed up to 10 kts as I have witnessed proper crosswind proficiency with this student.