Getting from Paris to Nancy

It doesn't have to be complicated. She can take a taxi to Gare de l'Est (it's a fixed fee- €55 I think), then a direct train to Nancy, or a train with one change en route. It's probably not a good idea to book the train ticket in advance since there are all kinds of things that could delay her and cause her to miss that train. Instead, just buy a ticket at a ticket window for the next train that's leaving.

Look up the train options and ticket prices on the sncf website: https://en.oui.sncf/en/train-ticket

What's the travel date? A quick look suggests that the transfer from Paris Nord to Paris Est will be required, but I believe those stations are just under 1/2 mile apart, so walking between them shouldn't be a problem unless she is burdened with excessive luggage. Depending on her timing, there may be only one other transfer.

It's a fixed fee of 50€ to Gare de l'Est. If taking the RER and walking for about 10 minutes (or taking the metro one stop) is too involved, just take a taxi to Gare de l'Est. If your daughter goes by train, I would suggest walking from Gare du Nord to Gare de l'Est. It takes just about the same amount of time as taking the train. I would not bother with going into Paris though. I would just take the train from CDG to Nancy. It can be done with one change which is no more difficult and probably is even easier than going into Paris and getting a direct train. FYI, currently, that one change could be from the train to an SNCF-operated bus if the connection is in Lorraine.

I would take a TGV to Lorraine from the airport, and the SNCF connecting bus.

I would do what Balso said. That looks like the least hassle and fewer connections/transfers to negotiate. You buy the ticket as one ticket.

Thank you all so much for your advice! The last two comments (Kim and Balso) suggested I go straight from Charles de Gaulle airport to Lorraine and catch a bus from there. But I am really struggling to find the correct website to buy one ticket (as Kim suggested) that will include the TGV to Lorraine and the SNCF bus to Nancy. Can you suggest the correct website?

The best website for that train-bus combo would be OUI.SNCF : https://en.oui.sncf/en/train-ticket . Official ticket-selling portal of the French Railways (SNCF).
Allow three hours from plane to train at CDG Airport, when coming from overseas.

Thank you Balso!