The prior work was a hatchet job. It will need to be repaired because it's improper and unsafe, as it is.
It looks like either the original black wire in the left cable broke off deep inside the box - but noting the ground wire is also destroyed, it's more likely the ass-hat who wired this just cut the wires off, because they think that's an appropriate solution to wires they don't know what to do with. /facepalm how unnecessary. But regardless, they obviously ran a second cable to the heater from another source, and are somehow using that. Can we save it? Maybe.
The simplest answer is rewire the whole shebang but for the sake of the drywall, I would advise carefully surveying which wires are available, both at the thermostat and the heater. Are you entirely sure that the intact black/white pair is from the supply? (if so that's unfortunate.)
Can you get us some pix of the wires at the heater? The only thing that makes sense is they also ran power to the heater...
If power indeed goes to the heater, I wouldn't be afraid to re-task the white and damaged black wire for a 24-volt thermostat circuit. which would give us a chance to rearrange this thing to be 24 volts, by adding a transformer-relay such as a Honeywell Aube - assuming an audible click is tolerable. If the guy didn't snip off the wires we need.