I've been doing taxes a long time. Where I work, we don't see many actual subpoenas served on a client. When we do, we refer them to a tax attorney with knowledge of criminal matters.
I suggest you find one of those before saying anything to anyone; even if the facts don't directly involve you, only what you know.
When you are a Bear of Very Little Brain, and you Think of Things, you find sometimes that a Thing which seemed very Thingish inside you is quite different when it gets out into the open and has other people looking at it.
--W. T. Pooh (aka A. A. Milne)