If she can convince the Jobcentre that she is "habitually resident", which usually involves between one and three months of physical residence, then she can get income-based jobseekers' allowance. If she gets JSA(ib), then she is automatically eligible for housing benefit. So it would be in her best interest to go to the Jobcentre and claim as soon as possible, and to continue claiming every week until it gets accepted.
It might help if she can show documentation that she has uprooted her life in the US to make the UK her permanent home, since that's one of the factors in determining habitual residence. But, since there is no concrete definition, just some court decisions, it will really be up to the person deciding her claim.
Or, if she takes any little job, she becomes a worker under EU law, which makes her eligible for housing benefit if her income is below the threshold. So even if she can work 10 hours a week at Starbucks, it should be enough to get her into a place.
(see
http://www.housing-rights.info/02_2_4_HB_eligibility.html#EEA-nationals, specifically the parts about workers and JSA(ib) recipients being eligible)