I'm driving around and I stop to ask someone for directions to my destination, is there an English word for that person?
Since the question does not specify that the person actually be helpful, just someone who is stopped to ask a question of, they could be a "bystander":
bystander: a chance spectator
or more specifically to your question, assuming whomever it is you stopped was on foot, they can be a "pedestrian"
or a "pedestrian bystander" :-)
They could also be called a guide if they actually led you to your destination, or to some point closer to your destination.
guide - a person who advises or shows the way to others
I don't think there's a specific word for someone who gives directions per se. However, we often use the word stranger when talking about someone we approach but have never met before. (We might be asking for directions, or advice about a good place to eat, or some other request, like "Would you take my picture?")
In other words, I could see myself telling a story, and saying:
We got lost, so we had to ask a stranger for directions.
If the people actually provide good directions, then they could be called helpful strangers. If the people are not familiar enough with the area to help out, but they are still courteous, they could be called polite strangers. If they are simply rude and unaccommodating, there's a word for that, too – but I'd rather not say that vulgar word here.