I'll tackle the "girlfriend investing" part.
First, the vehicles:
Roth IRA (Vanguard), deposit $6K for 2019 and another $6k for 2020.
Solo 401k (Vanguard) - $19.5K for 2020 employee side, plus employer side contributions (which can be much larger).
Second, the investment:
If she wants 100% VTSAX, that's fine if she has the mental fortitude to ignore the market gyrations. If not, I'd ask if she's ready for both the long-term gains, and the 50% drops (ie, she puts in $100k and it becomes $50k).
Otherwise, a target date fund is fine.
Once she's up in the ~$250k range, probably start diversifying to international, possibly bonds. By then she should develop an IPS.