The cable is probably not deep enough to harm the enough of the surface
root system, but if you are in a drought watering will help. I would
bet weed killer and other things hurt your trees more. If they cut 6ft
down then you would have real reason to be concerned of future die off,
but at 15 ft it may be ok. Talk to an arborist, either way watering
during drought and heat helps old trees since maybe 40-60% of a trees
roots are near the surface. If I was you I would be mad at the cable
company.