There are many reasons why you might want to send handmade friendship cards. They can be sent for special occasions such as birthdays, Easter, Halloween, when someone has moved into a new home, completed a course successfully, is having a bad time, or just because the person is your friend. No other reason is really needed. The real reason is because you are thinking of them and want to let them know that.
Also there is a friendship day that is celebrated in some areas. That of course would be a good time to send all your friends a card to let them know you are thinking of them on that special day.
Since there are so many occasions when you could send handmade friendship cards, there are no particular types of cards that you need to send. You will choose the type of card depending on why you are sending one. It may be one that expresses happiness for her accomplishment, or wishing her happiness on a trip she is about to take. It may be one expressing condolences which might be almost automatic if she has lost a person to whom she was close, but it might also be because she has lost a pet. It may be get-well wishes or offering encouragement that the unhappy events in her life will surely improve in the next while.
Whatever the reason, you will be expressing your heartfelt feelings in a more permanent way than a telephone call does. A phone call is an excellent idea for any of these occasions but a follow up with handmade friendship cards will be make it even more meaningful. Your thoughtfulness will be more evident and she will have the card to remember that thoughtfulness for a very long time.