If my opponent kills a bird token to play Stalking Wolf, can he react to that by playing Friendship to give +2 to the Stalking Wolf?

No, the player casting Stalking Wolf cannot react to the sacrifice of the Bird token.

Ruling

No, you cannot react to the sacrifice of the Bird token.

Reasoning

1. Cost vs. Ability: According to Rule 357 and FAQ #10115, paying costs is a mandatory, integral part of the process of playing a card. When a player plays Stalking Wolf, they must pay the additional cost of killing a Bird, Cat, Dog, or Poro they control at the moment they initiate the action.
2. No Window for Reaction: Because killing the unit is a cost paid to initiate the play of Stalking Wolf, it does not use the chain. The unit is destroyed as part of the payment process before the card is ever placed on the chain. Therefore, there is no window of opportunity to react to the sacrifice itself.
3. Timing of Spells: By the time Stalking Wolf is on the chain and becomes a valid target for a spell, the Bird token has already been destroyed and the cost has already been paid.

Note: I could not fully verify this ruling against the retrieved rules.