Well if people are committing disability fraud, they SHOULD be reported. And if people are in a country illegally - not "undocumented" - they SHOULD be reported.
Those of us that do things legally and by the book already have enough hoops and costs associated with it. People that break the law and circumvent the process just make things harder for the lawful people trying to do the right thing.
In the past, the laws favoured those who lied, over those who are honest, but now they don't.
e.g. In 2008 the then government decided to bring in medicals for claimants, instead of continuing the 'trusting people to be honest' on their disabled or too sick to work, benefit claims; law changes came in so that past visa appliction forms are now also being checked to look for those who were happy to use deception for a visa in the past and laws changed to remove these from the UK too now and ban them for at least 10 years.
Computer systems were brought in that can look for fraud, such as HMRC's Real Time Information system, the banks' National Hunter that the credit agencies operate for them; government departments now sharing information etc
Between that and neighbors reporting disabled people as benefits fraudsters just for existing and occasionally doing things with friends/family I'm already beyond upset with the tone of life here.
It's likely their own family that is reporting them. Some of the videos used to convict benefit fraudsters, can only have come from family. The first the claimant knows about it is when the DWP call them in for an IUC (interview under Caution) but by then the DWP already have enough evidence to convict them for fraud. The interview is for their side of the story and to see if they continue to lie. They can bring a solicitor to that taped IUC.
There was upset when some claimants with a false disability claim, realised that other people can now see that their car has been provided for their disablity (via a firm called Mobility) as before this would not be known to anyone. These cars, finaced by the taxpayers, are meant to be for those who can't get themselves over something like 20 paces, but this can be faked at their medical. Apparently the free Check a Car is Taxed online site, shows the car tax is exempt as they have a car from Mobilty and that only takes seconds to check.
https://www.gov.uk/check-vehicle-tax