Does the Heroic trigger from Ordeal of Heliod happen before Magma Jet, thus saving Favored Hoplite? Or does Magma Jet kill Favored Hoplite, with Ordeal of Heliod no longer having a valid target?
Favored Hoplite won't be saved. Ordeal of Heliod will have no valid target, will be considered countered, and will be sent to Player 1's graveyard.
When do Heroic abilities trigger?
Heroic abilities always follow this format:
They trigger right after a spell is cast. A spell is considered cast when you place it on the stack, having paid its costs, chosen its target, and so on. This is the point when you and other players can put other spells and abilities on the stack to respond to that spell. It's also before the spell resolves. (Tangentially: even if a spell gets countered, that doesn't change that it was considered cast at some point. It just doesn't get to resolve.)
So you cast a spell targeting your Favored Hoplite, triggering its ability, which goes on the stack on top of the spell. The Heroic ability resolves before the spell does:
However, these haven't resolved yet. At this point, before either resolves, you and the other players have an opportunity to place other spells and abilities on the stack. If you're confused about how the stack works, check this question: In plain English, how does casting spells and using creature abilities work with the stack and priority?
What happens in this scenario?
In this case, your opponent may immediately cast Magma Jet (since it's an Instant) before either the Heroic ability or Ordeal of Heliod have a chance to resolve. That would make the stack look like this:
Magma Jet will resolve first and kill your Favored Hoplite before the Heroic ability or Ordeal of Heliod get to do anything about it. You'll need to find another way to save it. That could include casting any instant targeting Favored Hoplite, which will trigger its Heroic ability again - and that one will resolve before Magma Jet and save it.
When in doubt, build an actual stack. Physically use your hands as placeholders for triggers. If the stack is too large, find pieces of paper and write down the name of what triggered. You don't have to be a programmer to build a stack out of of paper.
You cast Ordeal. It goes on top of the stack. Casting Ordeal triggers heroic. The trigger goes on top of the stack. You pass priority. Your opponent casts Magma Jet. It goes on top of the stack.
Now your stack looks like:
You both let the stack resolve at this point.
The top of the stack resolves. Magma Jet kills your guy. Next, the top of the stack resolves. The heroic trigger has nothing to put a counter on anymore, so it doesn't do anything. Finally, the top of the stack tries to resolve, but Ordeal has no legal target anymore, and so it gets countered by game rules.