If he's confused about how to step back without being "rude", here's a step-by-step.
First, he stops picking up her calls at least half the time. If she questions why he's not answering, he simply says, "I was with my wife/with my son."
Next, he pulls back on the text messaging. Exchange messages, sure, but not more than once every couple of days. Even best friends and family members don't need to message or call each other daily. Just stretch out the time between when he receives a message and when he replies.
If her expectation is that a married man will text her every day, multiple times a day, in order to maintain their friendship, that's 100 percent an issue with her expectation, and she needs to reset her expectation before she causes issues in another marriage.
And if she brings up "Why aren't you texting me as much?" he can simply say, "To be honest, our level of contact was starting to bother my wife, and when I thought about it, I could see why, so I've just pulled back a bit. It's not that I'm upset with you, and it's not because there's anything inappropriate being said or done, as of course there is nothing more than friendship here, but I think she was right that it was a bit too much contact with a woman who's not my wife. I'm sure that as a woman, you can also understand!"
If that's too 'real' for him, he can just say, "I've been real busy with family stuff lately, sorry! My baby boy is starting to become real active, so my wife and I have had our hands full."
The important thing is that he establishes you and him as the priority unit in her eyes, rather than him and her. And if he refuses to do that, that's something you will need to think about very seriously.