What I would do is tell your work you need to get from NYC to SFO then back to London.

Don't say anything to BA. When your friends check in, they just need to say that you've been called away on business.

However, I suspect you're going to find the cost of NYC-SFO-LON being way higher than DUB-SFO return. Suspect you booked this in a sale, but the cheap WTP fares I can see from Dublin at the minute do allow for a change for EUR180. Problem now is that the fare is going to be way higher - looks like WTP from DUB to NYC is circa EUR1500 pp.

I would really advise against doing back to back transatlantic flights - you'll be a wreck by the time you get to California.