"Farewell!"
farewell party n.
"Today, we're saying farewell to him/her."
"It's not goodbye, it's see you later."
"You're going to be missed."
"I'm missing you already."
"You're/he is part of the furniture."
"I heard you're leaving us."
"Stay in touch!/Keep in touch!"
"Come back and visit!"
"Come and see me!"
"I'll come back!"
drift apart v.
"People can drift apart."
lose contact v.
"People can lose contact."
"You must keep your end of the bargain."