So on Sunday 24th and the week prior 17th The Queen and DOE attended the Polo Matches at Windsor.
Unfortunately nobody can tell you for certain your friend is lying, as we weren't there. You could ask him what time he attended the service, here is a timetable of services (https://www.stgeorges-windsor.org/wp...4-18.07.07.pdf
) that were held on 24th, it would have had to have been the morning service for him to see The Queen.
There's no breach of security, I do somehow doubt he was allowed to knowingly sit next to The Queen, perhaps close by. Services are open to the public and The Queen knows that, it happens often in churches in Scotland.
There is also no "Windsor Chapel", it is St Georges Chapel attached to Windsor which The Queen attends.