Firstly, tell your friend that it’s normal for men (even married) to have the desire to look at inappropriate materials. Even the most frum men can have such a desire, and she should not think that her husband is abnormal/ weird. The problem arises (besides basic halacha) if this desire is preventing him from being a good husband and father, which may be the case here.
Second, her husband probably knows that he’s doing the wrong thing by looking at inappropriate materials, it’s just that his temptations got the better of him.
Third, if your friend is worried about her husband, she should tell him what she found and talk to him in a non threatening way. She needs to understand and clearly tell her husband that this is “his” problem which he alone must deal with and figure out how to solve.
But, it’s not her job to be his policeman and snoop on him. She should continue to love her husband (hopefully she still does) and pray that he fixes himself up (and it would be good to get rid of the internet at home) before he destroys himself or his family.