What is the ruling on a wife retweeting messages of love and poetry directed to her by her husband on Twitter, and what is the way to deal with this behavior?

There is no harm in a husband and wife showing affection to each other in front of others, as the Messenger of Allah, peace and blessings be upon him, did so, as narrated in Tirmidhi's Hadith from Amr ibn al-'As, may Allah be pleased with him. However, customs and traditions should be observed. If uttering words of love is considered immodest according to people's custom, then it should be avoided, as Al-Hafiz Ibn Hajar pointed out in his commentary on Ali's, may Allah be pleased with him, not asking about madhy (pre-seminal fluid).