Does shunning include shunning someone who was committing a sin, and then this person asked for the friendship to be re-established? And is not accepting this friendship after he initiated the greeting considered blameworthy shunning that prevents good deeds from being accepted?

If a person repents and abandons a sin, it is impermissible to boycott him because repentance expunges sins. The boycott is lifted, according to the majority of scholars, merely by greeting him and his returning the greeting, in contrast to Imam Ahmad, who believes it is not lifted until the person returns to their previous good state. One should cease boycotting this person unless an evil or harm becomes evident from him. However, if he returns to the sin, or invites to it, or if no rectification becomes apparent from him, then he must be boycotted.