I used to work in this field and even contract/agency workers have rights, regardless of how long they have worked for the company. If your friend works as a direct employee and her performance is not up to standard, she should firstly be interviewed by line manager and given a verbal warning, together with a clear explanation of what is going wrong and what is expected of her. If there is no improvement, it then moves to a written warning. After that, failure to improve can result in dismissal, but the employee should be given notice as per whatever the contractual obligation is.
For agency workers, the Agency Worker Regulations introduced a few years ago, give contracted workers the same rights as permanent staff after a period of continuous employment - two years for most things, including pay and flexible working hours. However, the agency itself is responsible for its’ employees. If the contracted employer has issues with agency staff, they will report their concerns to the agency and then it’s normally an agency rep who will deal with them. As far as I am aware, the disciplinary procedures for performance/attendance issues for agency workers are broadly the same as for permanent staff, so regardless of length of service, they should be given the same verbal and written warnings before there is any move to dismiss, and they should work out their contractual notice. Instant dismissal is usually only for gross misconduct.
Having said all that, my partner spent some time in the employ of an agency, contracted out to a multi national corporation and at times it was a bit fraught. He says there were a few occasions on which fellow agency staff were summoned from their desks and escorted off the premises, never to return, so it depends on what they term as gross misconduct. It might be worth contacting ACAS regardless of whether she is an agency worker or not, as what’s happened, definitely doesn’t sound right if it’s entirely performance related.