There's no "exact" penalty as far as I know ....but I'm not a lawyer / expert and I'm answering because I probably have a *slightly* better knowledge than many folks on the board. Here are some of the considerations.
a) Penalty under law for driving "otherwise than in accordance with a license" - 3 to 6 penalty points on your license (yes, I know, she doesn't have one
) plus a fine. Not sure how much that would be but it would be up to the courts to decide, and they're notoriously patchy in what they hand down.
b) Unless her insurance company knows she's driving without a valid license and accepts that (which I doubt very much), she may be liable for the full cost of any accident and that could be VERY expensive if someone gets injured or worse. In such a circumstance, she could also have a further court charge against her of "using a vehicle uninsured against third party riskes" - 6 to 8 points and a further fine
c) I don't know how much problems with the law / courts in (a) and (b) would effect any future Visa application ....
Please suggest to your neighbour that she (although it's a hassle) she would be best advised to follow the rules of the country she's chosen to live in. It could be regarded as arrogant if she chooses to adhere to only the rules that suit her here in the UK, and courts, immigration and insurers may give her an especially hard time if she gets caught.