That is awesome, congrats to you both! Now some honest advice.

Get her a trezor or ledger

Reasons:

1. Security: this one is pretty obvious. Even with MFA it is entirely possible for someone to steal your phone and/or put a virus on your PC etc.
2. Privacy: coinbase has already lost a suit for 2013-2015 to provide account holder information, even in cases where there is no evidence that any coins were sold or that any capital gains were realized. Even if you follow every tax law to the letter getting audited is a huge hassle that you don't need.
3. Free money: when you have that much coin the value of forks becomes significant. As an example, trezor owners got access to their BTG at a time when the BTG/BTC rato was ~double what it is now. By the time coinbase users get access to BTG it likely will be nearly worthless. The value of the early access to BTG is worth thousands of dollars if you have 6BTC, more than enough to cover buying 10 trezor/ledgers

Congrats to your mom and best of luck!