It is impossible to provide a definitive answer as the word "reasonable" does not have any definition and is dependent on the specific situation. However, if the retreat is optional and could be seen as a "privilege" I don't see that a it would be reasonable for your to incur these charges, the fact that you are allowing the carer to come along for free means you aware not costing the individual any additional costs to come along to something which is not a necessity. In realty, if they felt this was not acceptable, it would require them to take the matter to court and given this issue the cost of doing so would likely far outweigh the rewards. If the retreat was "mandatory" that may be a different story, however as it would be a privilege I don't see it would be a legal necessity for you to incur these costs. For example, if they wanted to go on holiday, it would not be up to the holiday provider to cover the costs of the carer, but offering free travel for them would not be seen as reasonable. I hope that makes sense.
I shall provide you with a helpful link that will assist you in finding a solicitor/representative near your local area. This will provide you with someone nearby your area who can assist you if required.
I hope this information proved helpful. You will find a local solicitor who deals with these matters on the law society webpage which is as follows;
https://solicitors.lawsociety.org.uk/ (England)
https://www.lawscot.org.uk/find-a-solicitor/ (Scotland)
https://www.lawsociety.ie/Find-a-Solicitor/Solicitor-Firm-Search/ (Ireland)
https://www.lawsoc-ni.org/solicitors (N. Ireland)
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