I am sure your friend is correct that building regulation approval would have been needed, to remove what sounds like a large part of the main exterior wall, to ensure it was supported correctly and complies.
There are two things that spring to mind. One is to let your Solicitor take out an indemnity policy that would cover your buyers, if action were to be enforced by the local building control, or if they were to any take enforcement action in the future. I believe they cost roughly a couple of hundred pounds.
Or apply for a regularisation notice from your local authority building control, this is a retrospective consent that the works comply with building regs, but could be subject to an inspection which may involve stripping back plaster, to ensure the works are compliant. This involves a fee and can take time if you are wanting a quick sale, it may not be appropriate.
A word of caution though, once you have notified building control about your concerns, you are not allowed to take out the above indemnity policy, so be sure not to involve building control, if you do want the above indemnity policy.
It would be good to get your Solicitors views and advice on this, before doing anything.