In terms of the compositional characteristics, I think you want to have a read of the Jam and Similar Products (England) Regulations, or equivalents for Wales/Scotland.
For the general labelling question, note that your friend is in the "pre-packed" category, rather than "pre-packed for direct sale" - the latter refers specifically where food is prepared on the premises for direct sale to the consumer (e.g. sandwich shops), and I'm guessing they aren't making the jam on their market stall?
I'm not aware of any exemption based on the size of the business / method of sale (other than those relating to pre-packed for direct sale, that don't apply in this case) that would preclude the need to label in accordance with Regulation (EU) 1169/2011, so yes it does look like they may need to have the usual legal name, ingredients list, nutritional info etc.
Nonetheless it's not an area of the industry in which I have any great expertise!
I did find this guide from Food Standards Scotland (which should theoretically be broadly applicable across the UK) specifically for farmers markets, that seems to support the labelling requirement: https://www.foodstan...lling_Guide.pdf
As for getting advice, I would go direct to whoever will be the direct regulator - this will almost always be the Trading Standards department with jurisdiction in the town/region where your former colleague's business is located. They're usually very helpful, and tend to appreciate people taking the initiative to seek advice in advance, rather than finding out about something when it's referred to them by a consumer or similar