Being struck by lighting will have been more damaging to the tree than the family of squirrels. If the opening is old, and the tree appears healthy otherwise, there is not cause for concern. The squirrels are just utilizing a natural cavity to stay out of the elements. It's warmer than a nest built out of leaves. You may notice that they line the nest with leaves and small branches. So if you notice dead leaves protruding from the opening, don't be alarmed. The tree is fine. Be aware that a severe lighting strike, especially one that creates an opening in the bark, has the potential to eventually kill the tree. The opening can provide access to insects and disease which will weaken the tree. Hard winters and dry summers can further weaken the tree. So keep an eye on the tree to see how it is doing.
