I wouldnt worry about any damage being done. It sounds like he came through the skipped dosage just fine. 
I would suggest you speak to your 9 yr old about his medications and start giving him some responsibility - in training so to speak.  I believe 9 is old enough to be a little more responsable to have ask the friends parent for his medication before bedtime.  We have to raise our CAH children to understand the importance of their medication and to begin early on asking them "what time is it, is it getting close to your medication time?".