Seven years is a tad short.
The Law Society have published guidelines for solicitors; I think most files involving property have a ten year minimum (recommended) storage period.
It is also good practice to send the client a 'file closing' letter, which would state that the file is now going out to archives, that it will be shredded in due course and that the client is welcome to come and take anything they like from the file before it goes out to archive. In this case, that would not have helped OP's friend, in that the friend was not the client.
The problem is, of course, that the siblings should have got independent legal advice when they originally signed the disclaimers (or whatever it is that they signed). But, having said that, unless there are particular circumstances (such as perhaps a pre-retirement clean-up, a move to a new office building, merger of practices, complete shortage of storage space) the destruction of a file after seven years is distinctly odd.