As mentioned up thread, the FCDO has been updated to say that the Belgian government have advised against non-essential travel in Brussels, but they haven't gone as far as to update it to 'them' advising it which is what would definitely trigger insurance.
It has been updated again this morning and now adds...
"Brussels-Capital Region is now at threat level 4 of 4: avoid all non-essential travel. This is a developing situation – follow the advice of local authorities."
So I think it is definitely possible that BA would re-arrange travel and that insurance could kick in. But you will have to ring them both to find out.

The BBC are reporting that the suspect may have now been shot. It is not certain it was the perpetrator, but if it is, then the OPs friends may have no problem, just carry on as normal.
Brussels shooting: Police shoot man in hunt for gunman - BBC News