Who is "email"?
If I understand correctly, your friend believes giving is taught in the Scriptures, but her issue is "when should the giving occur?" That is a question that has been much discussed and also formally debated. So, one can get a varying number of opinions.
Since giving is accepted as a command, it seems to me the main issue is one of Bible authority. Of course, Bible authority is a subject on which volumes have been written.
We understand that there are direct statements, or commands, in the Bible and most can clearly understand that form of authority. In connection with giving, we have a command in I Tim. 6:17-19. We are taught that following the approved examples of the apostles is authoritative (I Cor. 11:1; Phil. 4:9). We also know that based on information provided, we can reach conclusions. For example, in Matt.3:16 we are told that after being baptized Jesus "went up out of the water." We may infer that Jesus "went down into the water." These are the same ways we communicate with one another in everyday life.
When we approach the Scriptures with the attitude we just want to do what God has instructed and follow His will, understanding we will be judged based on the words He has spoken (Jn. 12:48), we will use the three approaches mentioned in the preceding paragraph. We will ask the questions about any subject--what does the Bible say directly? What examples can we find approved by the apostles? What can we conclude based on what is said? When we find information on our subject using those three approaches, we will use what is learned knowing we are following what God's word has instructed and thus know we are pleasing Him.
Now, when we search through the New Testament to try to find out when the Christians did their giving, we can find only one place where a specific day is mentioned. We can find lots of information about giving, the need to give, the purpose of giving, who to give to, etc. But, if we are doing our best to please God in every possible way, we will also search for when to actually give. When we have done that, we will find that the Corinthians were instructed in I Cor. 16:1-2 to give on the first day of the week. Yes, it does say "concerning the collection for the saints." We are to give for that reason because we have poor brethren, but we are also to give for teaching and other things the church needs to carry out its work.
When do we give for all this? We know that giving on the first day of the week is in the Bible, but we don't know about any other days because it is not mentioned. Just like the Lord's Supper is only mentioned as being partaken of on the first day of the week. What should be our conclusion based on the information given? If we give on the first day of the week, we know God will be pleased. If we give other days, we do so without any authority. Will God be pleased? We have no way to support that.
I know someone may say, suppose the church treasury is empty, and there is an emergency that must be taken care of on Tuesday. Do we have to wait until Sunday to give any money? My judgment is that we would immediately give in that rare circumstance to take care of the need. That would be like the ox falling in the ditch on the Sabbath. However, such exceptions should not become the standard. The standard should be doing what we know is approved.
I hope this is of some help to you in your teaching. Feel free to ask more questions if you desire to.