Upvote:1
The Other Documents section of the U.K. Visit guidance https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/673351/Visit-guidance-v7.0EXT.PDF states that βSupporting documents should back up statements made on the application formβ, therefore IMHO it would do no harm to include evidence confirming travel companions.
Whether it will make any difference to the outcome of the application is debatable, since you are not sponsoring the application.