There is an EU law in place, ratified by the UK, which states quite clearly what compensation your father-in-law is automatically entitled to - no ifs or buts, it is a legal entitlement enforceable in law/court. You can google "Passenger rights" and get the links and details - there are enforcement offices in most EU countries which can be contacted entirely free of charge, including in the UK.
In addition to the advice you already have, I suggest you include in your contact with BA a statement to the effect that if you do not hear from them you will proceed with pursuit of your legal rights but make it clear that in addition you/he seek(s) further compensation for exceptional hardship (if indeed you do).