No, no foot in mouth. You didn't know they ended it, and you probably would've said the said thing if it were any other girl anyway.

if this happen in the last couple days then your life sucks. if he was dumped a week before then you may deserve it

If he'd known, it'd be a YDI.

It's not your fault nobody told you, apologize and tell him you didn't know. Problem solved.

I don't see the YDI in this, how would OP just magically know why the person wasn't wearing his ring?

I am curious as to why the friend had an engagement ring. I mean it's not traditional for males to wear an engagement ring. So it is weird to notice that.

I've never known a man to wear an engagement ring.

Damn, I feel bad for him. how come you're finding out so late?

HIS engagement ring?

Yes...? Is this a problem?

Yes. It is. Men don't wear engagement rings.

11, most men don't, but nothing is wrong if a man wants to wear one.

It's not common, but it does happen.

This needs a follow-up.
Dependent upon what you said and how you said it you could have very well deserved. We don't know why or how you and your ex broke up. There is definitely more to this esp considering it's about an engagement between your ex and a friend of yours.

"Listen man I'm so sorry. I had no idea. How are you holding up?" (Very serious face here). Problem solved?
EDIT: #3 didn't see your comment there

It's not like you broke them up. If you weren't told about it then he can't blame you. Just apologize, you didn't know.