[**Edit: I'll leave this answer for interest, but it addresses a different situation where the blocks are supported by a force applied through two vertical walls separated by a fixed distance.**]<br>

The answer depends on the form of the friction force vs. applied load at each interface: [![Friction force vs. applied shear force][1]][1] <br>

The assumptions are that the blocks are perfect cubes and can only move vertically. At each interface between blocks there is a given normal compressive force that is the same for all interfaces and consequently a given maximum friction shear force that can be sustained by any interface. The actual friction shear forces (and the moments present) are different for each interface. The bridge fails only if the shear force exceeds the maximum value on two interfaces in which case all the blocks between the two interfaces (plus anyone standing on them) fall to their doom in the fiery lava! <br>
     
The answer depends on the form of the friction force vs. applied load at each interface. Two extremes are shown above. In the left picture, the friction force disappears entirely if the applied force exceeds the maximum. In the right picture, the friction force simply stops increasing when the maximum is reached. Real systems act somewhere between these two extremes. <br>

In case A, the blocks fall as soon as the shear force on one interface exceeds the maximum because the entire load suddenly gets transferred  to second interface which then immediately fails.  *So standing on one end of the bridge is a bad idea.* The force will be a maximum on that end interface which will fail and transfer the full load to the far-end interface which will also necessarily fail. <br>

In case B,  the blocks will fall when the shear forces on two interfaces both exceed the maximum. When the first interface reaches its maximum, the excess load will be transferred to the other interfaces.  (The moments across the interfaces will readjust accordingly as the forces shift.)  The system will not fail until a second interface also reaches the maximum load.  The total load (blocks plus person) cannot exceed twice the maximum friction force.  *It doesn't matter where the person stands*. 
 


  [1]: https://i.sstatic.net/XXwwA.png