I was reading from the election code and didn't see that. It's a minor point, though.
I can't access the Statutes website right now or I would have given you the cite from the Statutes. Somedays they make me pull my hair out making sure I've looked at all that are applicable.
My experience with poll watcher challenges is that the attorney you spoke with was possibly essentially correct. Poll watchers are limited to challenges they make at the polling places.
You'll have a difficult time proving that the voter in question was not entitled to assistance or that there was illegal influence on their vote. Not saying it didn't happen - it is just difficult to prove.
Good luck and good citizenship by being willing to get involved in the election process.