I'm wondering if i can have a weapon focus crafted and then spend the karma to have it bound to my ally spirit instead of me, so my ally spirit can use it instead of me. Can I?
SR5 Core pg. 318 "You can't use a focus unless you've bonded it to yourself (and you can't bond a focus unless you're Awakened)."
SR5 Core pg. 318 "While activated, a focus also has an astral form. If you use astral projection, you also carry the astral form of any active foci you have."
SR5 Core pg. 400 - Sapience - "Most sapient critters are mundane, but they are capable of Awakening and possessing a Magic attribute...and follow the same rules for magic as normal characters."
Street Grimoire pg. 201 - Step 3: Choose Powers - "Ally spirits start with... Sapience..."
Street Grimoire pg. 202 - Magic Skill Use - "Every ally spirit is considered a full Magician..."
The first quote indicates that you need to be Awakened. The third says if you're Sapient, you can be Awakened and use magic as a normal character. The fourth quote says Ally spirits are Sapient. The fifth quote reinforces (okay, flat out says) they are full Magicians.
Street Grimoire pg. 202 has a whole section on Enhancing An Ally. You'll have to rebind the spirit with a Ritual of Change, but you can add forms, powers, skills, spells and force. Ally spirits are full-up magicians, so there is no reason they cannot bond a focus if you're willing to pony up the Karma for them to do it.
The second quote up there is a little snag I can't find a way around. An astral projecting magician takes the astral form of an active focus with him. It goes on to say that if he deactivates the focus, he has to go back to it physically to reactivate it.
However, most ally spirits don't astrally project. They have one of three powers (SG pg. 201, again) - Inhabitation, Materialization, or Possession. The description of Materialization (SR5 pg. 398) is pretty clear that astral is the natural form, and the physical Materialization is created as-needed. Possession (SG pg. 197) is worded in a similar fashion.
Inhabitation (SG pg. 196) could be a True Form, which is no different from a normal spirit (it ends up with Materialization). Hybrid forms explicitly lose access to their Astral Form - I would take this to indicate that they cannot project either. Flesh forms result in spirits grounded in a body, but with the Dual-Natured property; it's a grey area whether they can astrally project. (As a long-time SR GM, I'd go with "No, you filthy bug spirit bastard.")
The point of all this is...
Yes, you can give an ally spirit a weapon focus... but they can't take it with them when they dematerialize. Don't leave it behind, because it serves as an astral link to the spirit, and by association to the magician.