You have it right - just put the 2 squares at the bottom of your picture on top as the first row. Make setting triangles out of new fabric. Then use her triangles interspersed with new white ones as the border.

You could start with 2, instead of 3, on point. It would be 2,3,2,3,2,3,2. Thereby using all 17 blocks.
Then perhaps the largest triangles would go in in the 4 corners, and the other triangles on the sides (4) and top and bottom (2). Or, if 14 of the triangles are all the same size, you could use 2 in each corner instead of one larger one.
I don't know the sizes of all the parts, so just taking a guess.

you are so wonderful to do this for your neighbor

How about printing a block with the quilt history on it to replace the lost block?

This looks like a sampler quilt I made many years ago as a class to make a quilt as you go quilt. It was done using the
Georgia Bonesteel method. It was a great method. As for the missing square you could make one that is similar to those you have, or, you could write the story of the quilt, put it on a fabric square you can run through the printer (maybe use a photo or two) and put it into the quilt. The blocks are lovely and the story touching.

This will be beautiful when done and so nice of you to finish it for her! I think the dark blue sashing makes the blocks pop. Please post when you are done, I would love to see it!

Looks like it is set diagonally.

I have no suggestions that hasn't already been stated. Bless you for doing this for her and her grandson. He'll treasure it I'm sure

The navy should be easy to match if you don't have enough there. I'd sash all the blocks...I think it would showcase each one better. Just my humble opinion. Lovely of you to take on the task.

I love the second way that you laid out the blocks. I had also thought of forming them into a central medallion. Then you could use as many of the triangles as you needed and you'd be free to add borders as necessary to make a big enough quilt. You are a very kind person to help this elderly lady complete her desire to complete something wonderful for her grandson.