I cannot find anything that would support this.

The coach may be correct. The FLEX must use a re-entry (if available) to return to the game, but it is not a substitution. For that matter, just because the DP plays defense doesn't necessarily mean s/he replaced the FLEX, but if the FLEX doesn't go out on defense, s/he has left the game.

Just think about the process. The pitcher was replaced; if it was a courtesy runner, no big deal, the pitcher hasn't left the game. But, it wasn't; no matter how you wrap it, you had a substitute (upon discovery), and an illegal one at that.

So, the pitcher had a sub; how does she get back in the game, then, if not a re-entry? Until/unless you find a rule that says it isn't a re-entry, that is the only way she can get in the game.

Just like the DP for the FLEX isn't a substitution, but the only way back in the game is a re-entry.

I probably didn't elaborate properly. Coach wanted to confirm that the DP could play anywhere on defense and I said that was correct. But then he wanted her to play defense for the Flex and I told him if that was the case that the Flex had left the game and if she came back in, it would require a reentry. I think I have the concept, but used the wrong terminology.

I usually think of the DP playing defense for someone other than the Flex as the player coming of the field as playing "bench". And at that point both DP and Flex are on the field on defense at the same time. I thought that's what he originally had in mind. Glad I clarified with him and he took a different path.

But if it is a substitution, it seems to be a legal reentry of #2 for #22. However, the OP confirmed the coach was specific that it was a CR in this instance.

However, 8.10.A.2 states that neither the catcher or pitcher will be required to leave the game for a CR. And 8.10.F.Exception allows for the pitcher or catcher to return when a CR is no longer available to be a CR. May there be a semantics issue here because it doesn't necessarily make an allowance for the DQ's offensive player actually being the CR? Yeah, probably so, but I believe the application to this situation is viable.

Nowhere does it mention the use of a re-entry and the rule specifically noted that the player had not left the game unlike the DP/FLEX where that issue is addressed.

Of course, if there was a legal sub available, none of this would matter. :D

So, the moral of this story is, maintain and use the line-up card.

I agree with both explanations, the first almost verbatim.