Git question on reviewing tests and contributing fixes

As a reviewer of a test, is there any way in which I can propose changes to
a PR beyond raising an issue and waiting for the original author to fix it?

I think it is likely I don't understand Git/Github here... (per usual)

Let's say user "bob" created a fork of web-platform-tests and submitted a
PR on branch "fooBranch".  I looked at it, there are some issues I've
marked.

Really, I have the time right now and I'd like to just make the edits and
send bob a PR to fix them.  Especially for getting new contributors up to
speed, I think this is important encouragement.

I tried:

git remote add bob https://github.com/bob/web-platform-tests.git
git fetch bob
git checkout --track bob/fooBranch

Then I did a commit, and

git push origin bob/fooBranch

This doesn't seem to create anything from which I can create a PR.

git push

by itself tells me I have access denied ( I can't write directly to bob's
fork)

I also tried from the Github web UI to fork bob's fork, but that doesn't
seem to work either.

Is there any way to do this?  Does bob have to add me as a collaborator on
his fork?

Thanks!

Brad