What is the legal (Sharia) guidance for a wife who discovered inappropriate conversations and pictures between her committed husband and girls online, and fears for her family and the future of her daughters?

Chatting between men and women is one of the steps of Satan. What the husband is doing by establishing relationships with girls via chat is a very dangerous matter. The wife's duty is to try to reform and guide him, and to involve people of sound judgment and righteousness.

Note that the evidence, pictures, and flirtatious talk that the wife keeps are considered a form of spying, which is impermissible. Furthermore, showing them to him might embolden him to openly declare what he was hiding.

The wife must be keen on keeping her family cohesive, as its disintegration would lead to her own loss and that of her children. This is what she should use to appeal to her husband. If he repents and returns, then praise be to Allah. But if he refuses, then patiently enduring his current state and praying for his guidance is better than the family breaking apart.