Since she won’t be there long enough to be vested, and she’s planning to use a Roth, I’d have her set up her own IRA directly rahter than doing a 401k.
I don’t think it is worth it to go through the employer for 1 month. Too much hassle and possibility of the funds being ‘lost’ at some point. I had a pension that I contributed to for 4 years but had to get to 5 to vest. It just sat there and finally I claimed it because there really wasn’t a good way to inform my kids that it even existed.