Plantain as in fresh produce, sold still in the skin? That he should wash (yes, I know very few people do
) and peel prior to consumption?
I tend to write each letter as required, but as a rough guess (not knowing your products, processes, site etc) I think I'd go with something to the effect of:
Dear (name),
We are sorry to hear that you have been unwell.
Unfortunately we have not been provided with specific details that would allow us to investigate the particular product in question, but if you are able to provide (batch number, date of purchase, whatever it is that you require for trace purposes) then we will of course look into this further.
Nonetheless we can confirm that we operate a comprehensive food safety and quality management system, and that our products and facilities exceed all regulatory requirements. We are certified to (BRC, SQF, etc if applicable) and are regularly audited by both internal and external parties. Our routine (product/environmental testing as applicable) includes (organisms etc if relevant) and we have had no adverse results.
Our business takes great pride in its products and monitors all complaints and consumer feedback as part of our process of ensuring continuous improvement. In this respect we can confirm that we have received no similar complaints.
As with all fresh produce, we recommend washing prior to consumption.
On the basis of the available evidence we do not believe that our products are the cause of your illness, so whilst as a business we would like to extend our sympathies we will not be able to assist with the medical expenses at this time.
If you are able to provide further details to allow a more thorough investigation, or would like to discuss anything further, then please do not hesitate to contact us.
Yours sincerely...
Obviously prior to sending any communication you should conduct as much of an investigation as you're able - general review of whatever testing and controls you have for whatever might be a reasonable period around the potential complaint date. If you're under genuine significant time pressure then you can always send an initial "thank you for bringing this to our attention, we are investigating and will contact you shortly" letter, followed by something more substantial.
N.B. "Hospital bill" makes me think you're in the US? In which case you may want your legal team to look this over too, or to check with a lawyer and/or your insurers - medical bills can get expensive very quickly...