Unless as part of a conversation you enter into some sort of contract where you guarantee to cure person As diabetes, I'm pretty sure the only thing they could sue you for if you didn't cure their diabetes, or if your advice to low carb led to some sort of injury, is negligence.
In order to prove negligence person A would need to prove that:
1. You had a duty of care
I can't imagine any court deciding that you have a duty of care in respect of medical advice to someone you are having a casual conversation with. Especially if you are patently not holding yourself out as medical professional. If you are planning on selling your advice, it might be a bit different. But normal conversation between normal people no duty of care.
2. Breach of duty
If a duty of care is found, it would be a duty to take such steps in the conversation as the reasonable man would take. All the advice that everyone else had given about saying it worked for you but everyone is different is really sensible advice - if you do that no one could think you breached your duty (not that I think any duty exists in the first place).
3. Causation
They will have to prove that the breach of duty caused them damage/injury/loss. Person A already has diabetes, he is going to have to show that he wouldn't have suffered any injury claimed, but for your substandard conversational skills and the injury claimed isn't just an unfortunate risk of life with a pretty big co-morbidity. That's likely to be very difficult to do.
In summary, I'd be amazed if anyone ever proved negligence. And, bearing in mind clinical negligence claims are usually rather expensive to bring (for person A's solicitor/their insurance company anyway) unless you are really obviously loaded why on earth would anyone bother trying for such a minuscule chance of success.
It's lovely you want to help people. Just help people as if you're a nice person & no one is going to want to sue you.