There is a distinct difference between Manifesting and Materializing.
1. A Manifesting magician doesn't have a physical body.
2. Adepts use astral perception, astral combat and mental attributes to fight astral entities that does not have a physical body.
3. Weapon focus can aid in astral combat against astral forms without physical body if the wielder use astral perception or astral projection.
SR5 p. 314 Manifesting
Unlike the Materialization power of spirits (p. 314), manifesting doesn’t give you a physical form, so you can’t interact with anything physically (or be harmed by physical attacks).
SR5 p. 315 Astral Combat
Astrally perceiving and dual-natured characters use their physical attributes and skills to fight opponents with a physical body, and their Astral Combat + Willpower to fight wholly astral entities...Astral attacks with a weapon focus are an Opposed Astral Combat + Willpower [Accuracy] v. Intuition + Logic Test.
SR5 p. 320 Weapon Foci
A weapon focus is effective against astral forms too, whether you’re using astral perception or projection. It adds its Force to your Astral Combat Tests when you’re using it, and you can take it along while astrally projecting.
If the adept had astral perception (which he in this case does not) then it would be resolved with an Opposed Astral Combat + Willpower [Accuracy] v. Intuition + Logic Test where the weapon focus would act as a positive dice pool modifier and where the damage would be based Charisma rather than Strength.