He may have thought it was best to stay out of hot water.

Maybe he didn't know you well at that time to share such information.. or thought it would be unprofessional and inappropriate to share this. Now you've become friends and he's more comfortable with you.

Maybe it was a gender issue with regards to feeling emasculated and was too embarrassed to share.

Maybe as revenge, he behaved in a passive-aggressive way towards this boss by not completing the job.

Maybe he filled in for you without being given a say and having to work with this boss caused him further stress.

My point is, you can't assume. Abuse is abuse and when in that situation, we often don't behave "well" due to fear.

I wouldn't say anything more about it.