I think you need $25k to be a day trader on Robinhood. So if it drops below that he can't trade the same way he would unless he adds more cash which I guess he isn't prepared to do.
You can trade with less. You can't day trade, defined based on the number of trades in a certain period. Some brokerages may enforce additional limits, but I've personally traded on NASDAQ via US brokerage accounts with less in them than that, so you certainly can do it.
Day trading has a specific definition in this context (can't remember if it's SEC, FINRA, or some other acronym), and refers to the buying and selling of the same security with a single day. That $2.5k Fidelity account will go out of its way to tell you that you're not allowed to do that.
