Is she already in the US? Does she have US citizenship/green card/a visa with permission to work?
Whatever she does, make sure she's on the correct visa (even if the company involved is dismissive about it).
As Vim said, formal internships are usually on the J1 visa. But the US is also very restrictive about less formal voluntary work, which can rule out a lot of the more casual 'work experience' opportunities for overseas people. You can't work for free when the work would usually be paid. You can't accept any alternative recompense (including, I think accommodation). There are many grey areas.
If she sees herself eventually working professionally in the US, any misuse of the VWP, overstays, non-permitted work etc will really mess up her prospects.
This would also be an incredibly expensive undertaking, if she doesn't get on an paid program. Film/tv will invariably involve New York or getting out to the west coast; very high living costs. It will help her to build contacts, but if she's entry-level then I suspect that there's little to no chance that a company would sponsor her for a work visa, afterwards, even if they were really impressed with her, so she should be realistic and recognise that she'll probably need to build up experience and credits in the UK before the US becomes a viable professional option.
Echoing Vim again, a reputable US-based course might be better value for money, giving her both contacts and a qualification.
And, judging from my time in LA... nope, self-effacing is not a successful application strategy here!