import pandas as pd

sep = ' - '
df1 = pd.DataFrame([[1,"apple",1],[1,"banana",3],[1,"pear",2],[2,"lemon",3],[2,"orange",1],[2,"apple",2]])
df1.columns = ["id","product","seq"]
df1 = df1.sort_values(by=["id","seq"])
pd.DataFrame(df1.groupby('id',as_index=False)['product'].apply(lambda x: sep.join(x)))

df2 = df1.groupby('id',as_index=False, sort=False)['product'].apply(sep.join)
print (df2)
0     apple - pear - banana
1    orange - apple - lemon
dtype: object