Some of the above is true.
CGT is only an issue if the house if sold for more than she paid for it, hence capital gain
If it was her primary residence then you can get help via personal tax allowances to mitigate the CGT liability. The annual limit is roughly £11k for everyone but as it was previously her primary residence then there is an additional £40k one-off allowance that she is entitled to. It also depends on time scales as the Inland Revenue will give you up to 3 years of unnoccupied or even as rented. They will pro-rate your bill depending on the time she occupied the property.
At the end of the day she is liable for sale price – purchase price
but she can also offset work done on the house in terms of serious improvements e.g. fitting a central heating system, an extension
The HMRC website has lots of inpenetrable documentation but your best bet is to go to a tax specialist and pay them £500 or so to submit this for you.