Edit:
I reread your post and noticed you wrote that she knew nothing about him. That makes it kind of hard to grieve for a person. Grieving, at least in my opinion, presupposes some kind of connection. But it's hard to have a connection if the other person is completely closed towards you. What I wrote below still applies though. However, rather than grieving for the actual person, the feeling would probably center more around the ideas and speculations that your character had about this mysterious helper and about being alone again. Now she'll never find out about his true feelings for her or about who he really is. It's easy to build up an idealized fantasy about someone you lost. She would (could) then mourn the life she wished they would have had but that's gone now, forever. The thoughts are more along the lines of "how will I handle this without him?" "I wonder what he would think about this" "I wonder if he would be proud of me", something like that. But still, it's much harder to have someone feel deep loss and grief about someone they didn't really know.
Original answer:
Well, everybody grieves differently, I guess, so there is likely no universally True Way.
That said, it's generally better to rely on your own feelings for descriptions rather than following some recipe that someone posts here.
Maybe try to relate it to a less intense form of grief or pain that you have felt. The loss of a pet or a really good friend who moved away. A break up. Something like that. Than really dig into that feeling. Get lost in it for a while. Try to intensity it in your mind, concentrate on the bodily sensation of it and the self talk that happens inside your mind. Then try to relate to that loss that your character is feeling.
Then again, plain old imagination might do the trick. Close your eyes and imagine the person (or animal) that means the most to you. Make it really vivid. Sight, sound, smell, touch, the whole thing. Ten imagine that person being killed, right before your eyes. Again, make it vivid. Stay with the feeling. It's not going to be nice but that's the point. Then imagine what your ordinary life would be like. All the situations where you would notice that they are not there. When you come home and they are not there to greet you. The place where they always liked to sit, now empty. The places you loved to go to together. Things you liked to do together. Mutual friends. Seeing their favorite food somewhere. Seeing someone in the street who reminds you of them. That sort of thing.
Anyways, if none of that helps: here's my take, based on my experiences. The pain is almost physical, somewhere in the middle of the torso. You want it to go away but you can't. The initial shock can be disorienting, you might feel dizzy, your sight might go blurry, your knees might grow weak. You might be at a loss of words. Maybe you'll feel empty inside, unable to accept or even process the situation. You'll probably be unable to do anything at the same level that you used to be able to. You won't be able to enjoy anything at all for a few days.
However, the initial pain will pass after a day or two, especially when you have someone to talk to and when you give in to the pain. If not it can take considerably longer. The worst thing though is the emptiness that this person left behind. You'll be reminded of them by the most innocent and banal of things. As I already wrote: places where you've been together, their favorite food, stuff they used to hate, stuff they used to like, etc. And every time it stings. Every time the pain comes back. Not as strong as in the beginning but bad enough.
This is a pretty pale description and might be hard to transfer into your story. As I said, it's best to draw upon your own experiences, even if you have to stretch yourself to get them.
