USA alien non residential tax

[bruffin]

My dd worked through Camp America last year from June to end of August at an american camp.
Today she has been advised by CA that due to a change of rules she has to file a tax return by 15th April to the IRS. She was paid about $900 in cash.
Will she have to pay tax on that?
She is going back this year and will earn about $2000.
She is a student here in UK so only minimal earnings at home

From I understand us tax system doesn’t have a personal allowance anymore so you can earn small amount tax free. It looks like she’d pay 10% rate. But there may be allowances but the us system is quite complex.

Thanks,