On Wed, July 7, we went to the bus stop for this bus at Etoile which we always take going and coming Paris. Well, here we were at 6 am for the first bus to find that there was a bus strike. YIKES. The choice was a taxi at $40-50 or... a young man seeing our anxiety said he would show us how to get the RER. First the metro tkt to RER A, then change to RER B for CDG. Then of all kind things, he went with us as far as RER B. He refused any money, and seems like we made a new friend in Paris. But it cost $25 each on the train which we didn't expect (?) Luckily the conductor took a credit card since we did not have enough euros. So for the 2 of us the fare equalled the taxi fare. And we made our flight with lots of time to spare. But we were not informed by AirFrance of the problem altho they had our hotel number.
Other than that, we had a grand time. We ate at the Jules Verne Restaurant, visited Giverny, saw a ballet LaSylphide, even saw Harry Potter movie (hated it). And in general walked all over. By the way, the outdoor cafe portion of La Coupole seemed to have gone downhill. Never again. Happily just a short distance is Le Dome cafe which is now our favorite spot for a snack, and the views are ten times better.
We spent an enjoyable 8 days.

I haven't tried it, but my understanding is that the RER fare includes both metro and RER, and that it is a little under 8 Euros. Can anyone else comment? So you should have paid a little less than 16 Euros total, which would be about $20 USD.

Is the Air France bus strike still on, or likely to occur on July 11 (the day I'll be wanting to take it)?

How much did the ballet cost? I am thinking of seeing that, but saw only 70 Euro tickets on the web site.

I think you're right, WillTravel -- somehow 7.75 Euros comes to mind. But then, maybe there was a fine because the tickets were bought from the conductor? I don't really know for sure.

Would he have had to buy separate RER tickets for Line A and then Line B?

Or did the helpful man do the ticket buying and perhaps collect a commission for his services?...

I would have thought that it's a flat rate to get to CDG from the center of Paris -- i.e. all transfers/changes are included in the fare.

But I'm not sufficiently familiar with the fare system to know. All I can remember is that I paid 7.50 or 7.75.

The fare is only around 8 euro so I don't understand that. Perhaps there was a fine if you bought from the conductor, but I don't understand that, either, as I would have thought you would get stuck paying upon exit if your ticket didn't work in the turnstile (I assume you did not have anything but a regular zone 1-2 ticket). Well, I don't know about this, because I would have thought you could just pay the extra on exit if your ticket was for the wrong zones, not there being a fine, but I don't know.

Air France can't call people when their airport shuttle bus is late or cancelled or on strike. First, it's a public bus, basically, anyone can ride on it. Second, even if you are an Air France customer, probably 99 pct of them don't take the AF shuttle bus so it wouldn't make sense for AF to call everyone flying on all of their planes in the morning. That is a lot of people.

RER tickets cost 7.75 Euros each. I have no idea what the $25 each on the train is about unless it was some kind of a penalty, as others have suggested. (I've never seen a conductor on the RER, but I suppose there must be SOME enforcement out there some of the time.)

The ballet was great and is usually sold out. We did pay the 70 Euros each. If you want specifics on the ballet itself, use a search for La Sylphide. Lots of good data.
The helper was an independent citizen doing a good deed. We have had similar good deeds done in Spain.
We showed the conductor our metro tkt and he said it didnt fly and asked for the fee which we didn't have. He said a card would be ok. He put it in his little gadget and gave us the receipt. I suspect nothing sneaky, at least I didn't then.
I've never looked into taking the RER so I was at a total loss. For 20 years I've been taking the metro in town or the bus to CDG. Live and learn.