My girlfriend is applying for a UK standard visitor visa online and wanted to know if she has to state she had private medical treatment (an unexpected accident) when she visited last time in the UK, even though it was a private medical service paid in cash. We didn't keep the receipts nor the medical records given to us, so would mentioning it might bring more hassle than necessary?
Yes, the form does ask about that* and you should answer the question in yes if you have had it.
It’s actually good that you paid the full bill and didn’t use their Taxpayers’ money. I would mention that.
* It only asks for that from applicants who answer that they have been to the UK previously. First time applicants won't see this question.