All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
Communicating with non-mahram women (women who are not permanently unmarriageable) lead a man to evil and corruptions as we previously warned in fataawa 199039 and 89990. Your husband has definitely committed a mistake by communicating with non-mahram women. He did well by closing his account and abstaining from communicating with them. However, this is not enough; rather, it is incumbent on him to fulfill all the due conditions of sincere repentance highlighted in fatwa 86527. He is also obliged to refrain from contacting his female colleague; this is prohibited as well as long as there is no real need for it. The fact that this woman’s husband knows about it is irrelevant and changes nothing.
Anyway, your husband should have hidden his sin from you; when a Muslim is plagued with committing a sin, he is obliged to hide it and repent to Allaah for it. He should not disclose his sins to others. Abu Hurayrah narrated that the Prophet said:
"All the sins of my followers will be forgiven except those of the mujaahireen (i.e. those who commit a sin openly or disclose their sins to the people). An example of such disclosure is that a person commits a sin at night and though Allaah screens it from the public, he comes in the morning, and says, 'O so-and-so, I did such-and-such (evil) deed yesterday,' though he spent his night screened by his Lord (none knowing about his sin), and in the morning he removes Allaah's screen from himself." [Al-Bukhari and Muslim]
Your remark, "If I were doing what he is doing, he would not be happy," is hypothetical; you should not concern yourself or your husband with it. You should keenly avoid all that which is prohibited even if your husband happened to commit a prohibited act; as this does not constitute a valid excuse for you to commit a sin. Moreover, the fact that your husband does not see anything wrong with his relationships with other women does not necessarily mean that he would approve your hypothetical relationships with other men because if he were to do so, then he would have combined this sin with another one, namely being a cuckold man, tolerating his wife's infidelity. There is no doubt that the believer should not accept any violation of others’ honor just as he would not accept it for himself.
In any case, we advise you to be patient and continue to give him advice and remind him of his Lord. You should do so wisely and gently and avoid all that may be provocative. You should also show keenness to beautify yourself for your husband and be a good wife so as to help him refrain from thinking of any other woman; and supplicate Allaah frequently to guide him; verily, he whom Allaah guides is the one who is truly guided.
Allaah knows best.