I choose Ramona.
She was the only know person Ghost told to wait for him to close up.
And Tariq was climbing down the stairs. My guess is that Tariq was still in the building after giving his Dad a second chance to retract his words.

Now, while Ghost was shot and he was falling down.
He had a frantic bewildered look, like a real shock. The least person he expected to hurt him, shot him.
Now that look can either be out of Love for dear one or surprise for an unexpected known victim/enemy from the past.

Three persons are in the spotlight for this

• Ramona
• Tariq
• Q

And the Person that shot him was already in the building...