It's also worth noting that they were playing on iPads with timed turns (so any time you spend deleting your friend list is time you are not spending thinking about your play). After getting messages pop up while already in the match you are doomed either way; if the turns were infinite they could pause the match while RDU deleted his friend list. Why in the world they felt the need to host the tournament on iPads is beyond me (as a game developer / tech person it seems incredibly stupid to rely on handheld devices especially when you know that the iPad version has a reduced feature set compared to the PC version; and it crashes more often as well by the way). Hopefully this is a tech lesson they will learn and avoid this situation in the future.
For some reason friend vs friend battles now have timed turns... which actually seems bad for both tournaments and casual play. So RDU had a choice between spending time on deleting his friend list instead of thinking about his play. You can compare it to SC2; you would need to dedicate attention and APM to deleting your friend list instead of macro & micro. At a competitive level thinking time is incredibly valuable in HearthStone and does win / lose you games. Blizzard should add a pause option if they insist on having timed turns in friend vs friend mode for the sake of tournament play (in SC2 how many times per tournament do we see a "pp" because something went wrong with hardware? I don't recall many tournaments without any technical issues whatsoever)
Messages during tournaments
For whatever reason this seems to be an ordinary thing where players receive messages during tournaments from their friends, I've seen it happen tonnes of times. But really this is something that always had the potential to ruin a match so hopefully tournaments can mature from this event and find ways to disable outside messages during match play. (dummy accounts with only opponent friends, some kind of offline mode, etc)
That's my 2 cents; hope it is a useful contribution.
<3 <3 <3