transitive verb in Mandarin:外遇李小姐
When you use the word 外遇, it is a noun. It should be王先生有了外遇。外遇的对象是李小姐。
有了外遇 is the verb phrase for the noun 外遇. The 3rd person need to be mentioned in a separated sentence. In this way, you know who is the one betrayed the marriage and who is the 3rd person. And the 3rd person's marriage status is not mentioned, it can be anything.
Headlines sometimes tend to use super compact expressions, which are not common elsewhere. Like 或 means 可能， 拟 means 打算. Here I guess is the same thing, even though it is not common to say so, it saves space .