Learn Arabic (Modern Standard): هو مريض الآن.

هو is the independent subject pronoun for 'he'. Because Arabic nouns are either masculine or feminine, هو can also translate to 'it' for masculine objects, but since the sentence is about being sick, 'he' makes the most sense here.

Where is the word for 'is' in this sentence?

Arabic does not use a word for 'is' or 'are' in present tense sentences. You just place the subject pronoun (هو) directly before the describing word (مريض) to mean 'He is sick'.

If the pronoun changed to 'she', how would the rest of the sentence change?

You would use the feminine pronoun هي (she). Because the subject is now feminine, the adjective must match it. You would add the feminine ending taa marbuuta to the adjective, making it مريضة instead of مريض.

What is the wavy line over the first letter in الآن (now)?

That mark is called a madda. It tells you to stretch out the 'a' sound into a long 'aa'. It is used instead of writing two alifs next to each other.