What do you plan to do over this long weekend? I'm assuming you will be staying mostly in central Paris, zones 1-2, where the majority of the major tourist attractions, and that's what my answer is based on. If however you plan to be doing some farther-flung day trips (like Versailles, Fontainebleau, Disneyland Paris, etc.) let us know, because the answer may change.
So with that being said, I think the most economical option in this situation would honestly just be to buy everything point-to-point. In other words, no pass. The CDG tickets will cost €20.60 in total, and then métro tickets are €1.90 a pop or €1.49 a ride if you commit to buying a carnet of 10 tickets at €14.90 total.
Otherwise, here are the pass options, and why I don't think they really any of them make much sense in this situation unless you will be doing tons of transit trips:
A 3-day Paris Visite pass covers unlimited travel in zones 1-3 and is €26.65, but that doesn't cover the trip to/from CDG. You'd have to buy 2 point-to-point tickets to CDG at €20.60 total, which would mean the total = €47.25. For this to be cheaper than buying everything individually, you would have to be taking more than 14 to 17 métro rides ("14 to 17" depending on whether you buy individual t+ or carnet tickets). Think about it, will you really be taking the métro that many times over just 3 days?
There is a 3-day Paris Visite pass for zones 1-5, which does cover the airport, but that is €65.80. Given that that's more expensive than the €47.25 option above, I don't think this makes sense unless you plan to do lots of traveling in zones 4 and 5 aside from the airport, which seems unlikely for a first visit to Paris.
This board's favorite option (and my favorite option), the weekly Navigo Découverte pass, is probably not ideal either. It's only €22.80 for a week's worth of travel, and includes the airport. But it's strictly valid only Monday through Sunday and there is a €5 charge for a card. Even if your friend got one for Saturday and Sunday (which, incidentally, you'd have to buy for her ahead of time from the Friday prior to the Thursday before) that would be €5 for the card + €22.80 for those two days + €10.30 for the Monday trip back to CDG = €38.10. For this to be economical, you'd have to be taking more than 9 to 11 métro trips on Saturday and Sunday, and any métro trips on Monday would be extra. Again, do you think you'll be doing this?
All that being said, sometimes I do the math, find it's more expensive to buy a pass, and then buy some kind of pass anyway, because it frees me from doing the mental math of "is this trip worth it/should I pay for it" and it means I can just get on whenever and wherever I want. But if you're really on a pretty strict budget and don't want to spend that much, individual tickets are the way to go.
