obviously she should never accept cold-callers with their spurious offers.
The payback is not now very good.
If interested, she can contact BG or her electricity suppliers to see what is available. It may be a bit cheaper from an accredited local independent, but if you pay for it, it will take about ten years to recoup the installation cost. If you have to borrow money to pay for it, the interest charges may wipe out the gains.
The "free" installations are from investors who want to put panels on her roof and receive the generation payments themselves. They will have a charge against the property, for 25 years or so, which future buyers or mortgagers may not like so may reduce the value of the house. Your mother will be "allowed" to use the electricity generated, it might cut her electricity bill by about £10 a month if she uses her washer and drier on sunny days, and has an electric immersion heater, but the investors will keep the other profits. There will be no free electricity at night, and insufficient on rainy days to run a kettle or electric heater.
As she is not the customer of the installers, she may not have much hold on them if they kick holes in her roof or damage the gutters or brickwork.
It is proposed that the subsidy will reduce by about 85% in a few months, VAT on the panels will increase from 5% to 20%, and extra duty will be charged on cheap Chinese panels, so probably the solar roof panel scheme will shrivel away. People who already have panels will however continue to receive their subsidy payments.
I have a feeling that some people will continue to buy panels outside the subsidy scheme, especially if they have big roofs and are roofers or scaffolders. A typical installation currently costs in the region of £6k.