What's the worst that can happen if you emailed them in advance to all candidates?

Call it an experiment. It would help people who have interview nerves, people who are really good at research etc. Depending on the role, it may be more reflective of how they are expected to work by giving more accurate than fast answers.

The Equality Act 2010 makes clear that reasonable adjustments that favour a disabled person over a non-disabled person do not constitute unlawful discrimination, so from the point of view that the person with autism may benefit from the adjustment at the expense of the other candidates, you're fine.
From a usefulness of assessment perspective, you need to ask yourself whether sending the questions to the non-disabled candidates will offer them an advantage that may render the interview pointless, e.g., if they can just look up the answers and regurgitate what they've memorised, will the interview have any value? What problem are you actually trying to solve by treating them equally? Equality does not equal the same.

I would say it is only necessary to provide the interview questions for the candidate that has disclosed the disability. By providing this candidate with additional support, you are giving all candidates an equal chance of success (equity v equality). I would argue if you provided the interview questions to all candidates beforehand that this would give the other candidates an unfair advantage.

In reply to Cat Jones:

I'm not sure that, unless you are seriously considering sending questions to all candidates for all roles in future, this is a fair 'experiment'. This person has a risk, declaring their disability, and the reasonable adjustments are there for them to have an equitable starting point alongside other candidates without a declared disability.

On a related note, I had an interview last year for a senior HR role for a charity and they sent me and all other candidates the interview topics ahead of time as part of their recruitment practice. I really liked the approach and thought it was refreshing and wondered why fewer organisations didn't adopt it. They also had a very progressive shortlisting approach where candidates submitted short written answers on topics and were selected based on those separately to their CV to eliminate bias.

www.autism.org.uk/.../employers-fact-sheet

How did this go?

In a previous organisation I worked at we had the same situation and decided to send the questions to all candidates. It did not mean we had 6 perfect interviews, we had 2 roles and in my opinion only 1 candidate was worthy of being appointed.

A reasonable adjustment should not give a disabled person an advantage over their non-disabled colleagues / candidates. Giving the questions in advance to someone who is autistic does not provide an advantage, as it gives the autistic candidate time to understand the question, whereas the majority of neurotypical folk, are able to decipher the questions in an interview setting, or ask the right question to aid their understanding of the question in that setting.

I’d also suggest giving a detailed explanation of how the interview is going to be structured.

"There is nothing so unequal as the equal treatment of unequals". Good judgement to give the questions in advance.

Great question. Sending out questions in advance is not any more advantageous to one person over another in an interview, unless you are using the interview to see how people cope with unexpected questions. The interview is not an exam and it operates on the assumption that you are both using the process to get to know each other.

In reply to Nichola:

"I’d also suggest giving a detailed explanation of how the interview is going to be structured."
A very good call Nichola, a social story to reduce anxiety :-)

A belated welcome to our Community, David ... how do you think the process panned out at the interview(s)? 

In reply to Steve Bridger:

Hello all and many thanks for the responses.

Our HR dept said that we should provide all candidates with the interview questions in advance although it is interesting that there isn't a ready answer for this type of situation in terms of policy.

As Sandra said - the purpose of the interviews is for both parties to get to know each other and not to put pressure on responding in the moment.

Although not successful the candidate gave a really good performance and was stronger than some of the others and we were thanked for the way in which the process was handled.

It does make me question why we don't give everyone the interview questions - but I guess that is for a different conversation.

In reply to David:

Thanks for updating us, David.