Can I get from Gare TVG at CDG to baggage claim in Terminal 1 without going through security?

Where is she flying from? Within the EU or from outside the EU?
It is not security that matters, it is customs. Customs is after luggage pick-up. The public arrivals area is after customs.

She's flying from SFO in the US. I remember that customs was prior to baggage claim upon arrival, but I can't remember if baggage claim is still within the secured area. I'm hoping to meet my niece at baggage claim if possible.

No, she will collect her luggage and exit the secure area through the green "nothing to declare" door (I assume). There is usually a railing outside the door where people gather to meet arriving passengers. You'll see driver's holding up little signs to meet their parties.
Will she have a cell phone to call you when she's at the luggage carousel?
Assume you know you will need to use the internal shuttle train to get over to Terminal 1 from the TGV station.

For international arrivals, baggage claim is always before customs. The CDGVAL will drop you off at the lowest level of the terminal and I believe that arrivals are at the highest level of the building. Have you considered having her meet you at a particular spot like the Brioche Doree at the arrivals level in Terminal 1?

Lisa, it is immigration (passport check) which is before luggage pickup. Logically, customs must be after luggage collection, as they may want to see what is in your luggage. The order is:
1) Immigration (passport check)
2) Luggage pick up
3) Customs (walk through green channel).
4) Arrivals hall (crowds of people waiting to meet other people, some with signs). This is the first area wih public access.
You will meet your friend in the arrivals hall.

^^ This is perfect. Thank you!!