import pandas as pd

input_dict = {"client_name":["Joe", "Bob", "Joe", "Sally", "Dan"],
  "item_purchased":["Apple", "Orange", "Tangerine", "Apple", "Pineapple"]}

client_df = pd.DataFrame.from_dict(input_dict)

apple_clients = client_df[client_df["client_name"].isin(client_df["client_name"][client_df["item_purchased"]=="Apple"])]
print(apple_clients)