RecommendAFriend.cs

public class RecommendAFriendService
{
    private INewAccountValidator validator;
    private ICustomerDirectory customerDirectory;
    private IReferralPolicy referralPolicy;
    
    public RecommendAFriendService(ICustomerDirectory customerDirectory, IReferralPolicy referralPolicy,
        INewAccountValidator validator)
    {
        this.validator = validator;
        this.customerDirectory = customerDirectory;
        this.referralPolicy = referralPolicy;
    }

    public void RecommendAFriend(int referrerId, NewAccount friendsAccountDetails)
    {
        validator.Validate(friendsAccountDetails);

        var referrer = customerDirectory.Find(referrerId);
        var friend = customerDirectory.Add(friendsAccountDetails);
        referralPolicy.Apply(referrer, friend);
    }
}