Shingles is caused by varicella (chickenpox) virus that's already latent in your nerves. Really unlikely that it was a "co-infection", it just reactivated because your body was busy fighting Lyme.
I'm not sure what you think "co-infection" means, but you just described precisely that. The reactivation of shingles meant that he was simultaneously infected with two different agents. "Co-infection" (for me at least) implies nothing about how or where the secondary infection came from.
That's fair. It's a really commonly implied thing among the quackier side of Lyme treatment that every tick is going to unload a clown car full of pathogens (used to justify even more unvalidated tests). But, by the definition, it's just co-occurring infections regardless of vector.