Even more WT quotes that back up the father's party(but in no way say you cannot have a party):
*** w91 8/15 p. 28 Do You Remember? ***
How does Jesus’ illustration of the prodigal son show what our feelings and actions should be when someone is reinstated in the Christian congregation? (Luke 15:22-32)
Our goal ought to be to imitate the father, who showed happiness at the return of his prodigal son. Hence, we should freely speak with the reinstated brother and encourage him now to make progress in the truth.—4/15, page 25.
*** w91 4/15 p. 25 Imitate God’s Mercy Today ***
Jesus’ illustration of the prodigal son brings up a danger that we need to avoid. The older brother did not rejoice at the prodigal’s return but was resentful. May we not be like that, harboring ill will over a past wrong or begrudging a person’s being reinstated. Rather, our goal is to be like the father, who illustrated Jehovah’s response. The father was happy that his son, who was lost and seemed as good as dead, was found, or came to life. (Luke 15:25-32) Accordingly, we will freely speak to the reinstated brother and otherwise encourage him. Yes, we should make it evident that we are showing mercy, as our forgiving and merciful heavenly Father does.—Matthew 5:7.
*** w98 10/1 p. 17 Imitate Jehovah’s Mercy ***
The parable of the prodigal makes it clear that Jehovah desires erring ones to return to him. Indeed, he holds the invitation out to them until they prove themselves beyond hope. (Ezekiel 33:11; Malachi 3:7; Romans 2:4, 5; 2 Peter 3:9) Like the prodigal’s father, Jehovah treats with dignity those who do return, accepting them back as full-fledged members of the family. Are you imitating Jehovah in this regard? When a fellow believer, who for a time was disfellowshipped, is reinstated, how do you respond? We already know that there is "joy in heaven." (Luke 15:7) But is there joy on earth, in your congregation, even in your heart? Or, as with the older son in the parable, is there some resentment, as if no welcome is merited for one who should not have left God’s flock in the first place?
Doesn't get any clearer to me. We should imitate the father's example. By following his example, we should throw a party for someone who is reinstated. NO ELDER CAN ARGUE DIFFERENTLY.