Ask Lemmygrad

Just ask them to name specific imperialist wars they have waged and then debunk them.

Every war Russia has engaged in started as a civil war caused the dissolution of the USSR.
Ukraine was a civil war and russia only entered because the west refused to hold kiev to follow through with peace agreements that they had signed.
Georgia was similar. After the USSR fell apart the government of Georgia was let go but the minority ethnic groups in the north didn't want to be ruled by the others. Fighting ensured Russia eventually stepped in and the separatists joined Russia.
So even if the wars Russia has engaged in have expanded their borders it is not because they are expansionist it is because the borders were dumb and the brake up of the USSR was done in a slipshod manner.

Tibet willingly joined China, the only real fighting was terrorist uprisings by former slave owners, armed by the CIA, who were mad about the end of the theocratic slave state.
The situation in Xinjiang is the same operation only instead of Buddhist slavers, its a wahhabist al-qaeda splinter group, like isis, who want to enslave women and behead all "infidels" just like is going on in Syria right now.
Obviously Taiwan and Hong Kong are part of China (no involved party disputes it and if the person you are talking to does just lay into them for being ignorant fools who have no clue what they are talking about) If the government of China wants more political control over them that isn't expansionism.