First, regarding the modem, connect a laptop directly to the modem and do a speedtest. If you're getting the speeds she is paying for, you're good to go.
Second, regarding a Mesh system. Yes, the Orbi's do have a great reputation, though I just installed an Asus RT-AX92U (AX6100) mesh system at my parent's house. It's two-and-a-half floors and ~5000 sq/ft as well, which includes a bonus room over their garage. They now have full wifi coverage throughout their house, including outside on the front porch, and even over to their other (detached) garage and deck. They are using the 2-pack (one router, one client). You can add more as needed.
I have a single RT-AX92U at my house (1650 sq/ft), and have full coverage everywhere, including the garage, front porch, and back deck/pool area.
Also, the RT-AX92U is Wifi-6, tri-band. Basically it uses the third 5-Ghz channel (up to 160Mhz) as the backhaul between the mesh system. In my use, it connects at 1Gbps, way more than you probably need.
Vizio P65-C1 (2016)
Onkyo Tx-NR656 (5.1.2 setup)
JBL Studio S-Center, JBL Studio S26ii (FL/FR), JBL N24 (LS/RS), JBL N24 (Overhead Atmos), JBL Studio SUB250
Nvidia Shield TV 2019 Pro / Fire Stick 4K