My daughter's best friend is getting married, and her prospective mother-in-law gave a Tea to introduce the bride to some of her friends. She did receive some shower gifts at the tea, and thanked each giver in person. Now her mother-in-law to be is wanting to know when she is going to write 'Thank You' notes for the gifts. I always thought that it was not necessary to write 'Thank You' notes for shower gifts if you were able thank the giver in person. She did write notes to the other ladies who helped host the Tea, and plans to write 'Thank You' notes for all her wedding presents; but not for the shower gifts. What is proper in this case?

Thanks