stuff_it if possible try to ascertain what type of experience the assessor has - for example, are they a nurse, or a physio, or a chiropodist etc? Make sure you note this information as it may prove useful if you have to go to appeal.
Be polite throughout but bear in mind however nice the assessor seems, they are not there to be helpful to the claimant for the most part. Also as
Greebo has says, be aware that you and the claimant will likely be observed before the interview starts. Any small talk such as 'was your journey to the centre ok today?' may appear innocuous but can be used to elicit additional information e.g. when asked about travelling, claimant stated they had no issues using public transport and travelling alone to interview.
Your friend could phone up and ask about recording but this may result in the interview being postponed until equipment can ve booked, and even then there are cases of claimants turning up for interview to be notified that the equipment is unavailable, or hasn't been booked, or is broken etc. Take notes and if you want to use your phone to record (may be essential if claimant has memory issues for example) then I recommend you have it recording in a pocket with it switched onto silent until after you are out of sight of the building.