Is the husband's action considered unjust when he breaks his promise not to marry or communicate with another woman, which was a condition before the marriage contract, claiming it was not documented in writing, while he is now communicating with a foreign woman?

A mere promise of divorce does not necessitate anything, and it is recommended for the husband to fulfill it. However, if this was a condition that was agreed upon, then it is valid, and the woman has the right to annul the marriage if he marries another, especially according to the Hanbalis. As for his conversation with another woman without need or without being safe from temptation, it is forbidden, and he must repent from it. You should advise him and forbid him from this evil.