FPP goes first with regroup actions - if he do not want to do anything, the shadowplayer takes his regroup action. If he pass, the shadowplayer reconciles, the FPP decides to move again or to stay afterwards.
You see, there are some possibilities:
1:
LR discards the web. Shelob is still in play. SP take next action....
2:
LR waits, you pass - > no more regroup special abilities possible. Shelob is still in play, but not stacked.
3:
LR waits, you stack,
LR discard web and stacked shelob