The setting: an alien crew has a shuttle landed on Earth in the 1980's. Their main ship is extremely damaged and beyond reach somewhere in the Milky Way. Their shuttle cannot escape Earth's atmosphere. They are in an unpopulated forested area to avoid encountering the locals, which would be both a galactic crime and unsafe. Earth's gravity is uncomfortable but livable. They leave their shuttle only in EVA suits after decontamination.
The crew has begun transmitting a distress signal, hoping it reaches a stealth satellite belonging to the galactic republic that Humanity doesn't know about. Humanity is seen as a primitive pre-FTL race and as such their solar system is analogous to a nature preserve. Nobody will come save the stranded aliens if they don't make contact. This galactic republic has done some research and common Human languages are in the catalog for the crew's universal translators.
Reptilians secretly in control of Earth's governments have intercepted the distress signal emitted by the shuttle, jammed it with satellites, and sent their men-in-black agents to capture (or eliminate) the marooned alien crew.
There is a human from the distant future of a parallel universe who notices the agents moving in on the shuttle. He has access to extremely advanced technology, has drones and a heavy mech suit designed for research and exploration of any environment. He is not aware of the galactic republic and knows nothing about the stranded aliens other than what his drones have revealed with their reconnaissance. He wants to warn them about the agents who are approaching them. How does he do this?
EDIT: some clarifications were requested. In the future man's parallel universe, there was no galactic republic, and in fact there were no aliens at all as far as they knew. All of his equipment was designed for the scientific study of parallel universes after it was discovered how to traverse them. The primary purpose of his drones are for scientific observation, to gather data from a multitude of angles, so they aren't really equipped to interface with alien machinery. The man also has a full-body EVA outfit worn within the outer mech suit that features a form of optic camouflage, allowing him to study things up close. The man cannot return to his home world at this time.
Since the alien shuttle began transmitting shortly after landing, I would give it only a matter of days before the Reptilians closed in on their location. Once their agents are in the forest, hours. Neither the aliens nor the future man are aware that the distress signal is being jammed. The future man's suit wouldn't have enough Delta V to escape Earth's orbit and disable all the jamming satellites anyway.