public FriendViewModel(Friendship friend)
        {
            Expenses = new ObservableCollection<Expense>();

            this.Friendship = friend;
            LoadExpenses(friend);
        }
 private async void LoadExpenses(Friendship friend)
 {
     IEnumerable<Expense> expenses = await proxy.GetExpenses(friend.Id);
     Expenses.AddRange(expenses);
 }