Maybe this is a big can of worms to open, but I'm interested in hearing your take on this; I've heard that Archduke Ferdinand was actually very pro-Serb and had much respect for the Serbian people which tended to mark him as a black sheep to the Hapsburghs and therefore expendable. I'm actually friends with Serbs that had him as Kumovi in their family and they consider him a sacrificial lamb for the Austria-Hungarian Empire.
I heard that Archduke Ferdinand may have been sent to Sarajevo on Vidovdan knowing that he would likely be executed and it would be a great excuse for Austria-Hungary to declare an ultimatum to Serbia that they could never possibly accept and therefore Austria-Hungary could feel justified in swallowing up a Serbia that had been very much exhausted by the Balkan Wars.
Do you think there can be any truth to this theory?
The Serbian Royal Police warned Austria-Hungary that there may be an attempt on Archduke's life if he came to Sarajevo on Vidovdan, but that warning went uheeded. Serbia had no interest in fighting one of the largest empires in Europe, especially in view of the effort and sacrifice of successfully kicking Turks out of Europe in the First Balkan War (1912), and then defeating her ally Bulgaria in the Second Balkan War (1913) that started with Bulgaria's unprovoked attack on Serbia. So, Serbia had no motive to kill Archduke. It also had nothing to gain. On the other hand, Austria-Hungary had a rich motive and stood a lot to gain by picking a fight with Serbia, especially at the time when Serbia's energies have been drained by wars.
Looking at the political map of the Blakans in 1914, the only obstacle to the so-called Berlin-Baghdad Axis was Serbia and its tiny sister kingdom of Montenegro. They were the only two countries that had native dynasties and were firecly pro-western. All other Balkan countries, including Serbia's ally Greece, had foreign royalty, mostly German and, in the case of Albania, Italian.
Although tiny compared to super powers of Europe, Serbia has doubled its size in just two years following the Balkan wars, and was becoming a local powerbroker. More importantly, Serbia has been a source of inspiration to other Slavs in the area who were still in German, Hungarian or Italian lands. The Croatian Illyrian (later renamed Yugoslav) movement was looking towards Serbia as a way of helping Croats attain their independence from Austria-Hungary.
Serbia and Montenegro therefore stood as a living proof that Southern Slav can not only be free but victorious. That did not go very well with Vienna or Berlin.
In addition to that, Bosnian Serbs (who were a 44% majority population at that time) were boiling with a desire to rid themselves of Austrian and Muslim domination, and to join their kin in Serbia and Montenegro. Besidres, Bosnia has always been a Serb land -- and edicts of Bosnia's earliest rulers leave no doubt of that.
An underground movement called "The Black Hand" headed by a Serbian colonel with alias Apis was actively recruiting the willing and able to fight Austria covertly by means that, depending whose side you are on, are called terrorist or guerilla. The Serbian government was not happy about this and saw Apis as a dangerous destabilizing factor that could hurt Serbia's interests, so Serbia had very reason to warn Austria-Hungary of the impending assassination attempt.
The actual assassination attempt on Archudke failed. A hand granade tossed into his car was thrown out only to explode on the street and hurt members of the motorcade security. Later, Archudke went to Sarajevo's hospital to visit the victims of the attack. That visit was obviously not scheduled and could not have been planned as a target opportunity by the Serbs. The assassin Gavrilo Princip (Printsip) happened to be standing at the corner where the official motorcade leaving the hospital made an unscheduled turn. Having slowed down to practically a crawl, he rushed to the car and emptied his gun killing not only the Archduke but his wife as well.
If Archduke had been a friend of the Serbs, as you say, he would not have been assassinated because all the Serbs had to do is wait for him to take the throne once Fraz Jospeh II passed away. I also seriously doubt that your friends had Archduke over for kumovi unless they were at the very top of the social pyramid -- royal and government. Maybe the Archduke was passing by some village and was caught in an impromptou moment to be part of some celebration, but that is a coincidence not a pattern.