I don't like it.
No it's not cheating.
But in my opinion, it's also not OK.
My cheating ex who always stayed in touch with every female friend he ever met and cheated on me with several of them may have me jaded for sure. But at the same time, I know what healthy relationships look like in other couples I know, and in my own behavior and past relationships with respectful, loyal people.
Husbands don't need female friends who only THEY are friends with who their wives don't like (and the cheater-lady-friends don't like the wives either of course). Friends who are cheaters and who only THE HUSBANDS talk to with their OWN PHONES which they hide and lie about to their wives.
This is not how happy couples behave. This is not how good husbands behave. Granted, you sort of forced him to lie by suddenly saying you didn't want him talking to his old friend. Granted it's a bonehead perspective for him to play the "I knew her longer" card when you're his wife. But dudes are dumb. Granted he should see that a woman who cheats all over the place isn't the best secret friend for him to have. But again. Dudes are dumb.
If he is the most loyal personality type on the planet (and you should know this about him if you married him), and he ALWAYS puts your feelings first, and he would HAPPILY cut their contact down to an appropriate level of transparency and appropriateness, then he can have female friends of every spectrum of personality including home-wrecking ho's. And you could get off your high horse, keep your enemies close, and befriend her too rather than leaving them in the secret danger zone together. This woman is NOT a safe secret friend for him especially now that you've been put in the position of checking up on "them" and their secret dealings, you're making the whole thing worse. She's loving it that he says, "My wife doesn't want me talking to you so we have to keep it quiet." Ick she gives me the creeps and your husband is hopefully just clueless, not mean.
Anyway. I have never cheated on anyone. Not even casual boyfriends. I was the most upstanding wife on planet Earth. I have LOTS of male friends. LOTS of married male friends. LOTS of male FB friends. Were any of the men I knew outside of my relationships men I talked to FREQUENTLY on my own cell phone? Nope. Just gay men and work peers. Were any of them secret? Nope. Would I have kept on talking frequently and secretly to any of them if they upset my partner for any reason? NOPE. Do I have the cell phone numbers of any of my married male freinds? YUP!!! How often do I use them? ALMOST NEVER. Why would I use them? Well a few months ago one of them was having his friend look at my bathroom for me to repair it so we group texted about it. Another one was texting my from a car his wife was driving because they were lost on the way to my house...I don't NEED to be bffs with anyone's husband. Never have. Never would. Out of plain respect for their wives.I would not send them anything I would not be happy letting their wife read. Private FB messages catching up once when reconnecting after years are on the up and up and again, I'd be happy for the wives to read them, there's not ANY flirting going on.
The two men I dated after my ex: Never would have been sneaky. It's a personality type. We all have old friends, but only some of us put their feelings before spouses.
Friends that are alcoholics are not the same as friends who are cheaters. If I had a male freind who slept with lots of people's wives and girlfriends, NONE of my boyfriends or husband would have been OK with that. Unless he was just an acquaintance I saw in passing. A secret confidant in my cell phone??! Of course not. Even my cheating husband, Mr. Social, would not have stood by for that. He was jealous of my decent male friends even though my behavior was squeaky clean.
Is your behavior squeaky clean? Your husband's isn't IMO. But now you have a sticky mess to get out of or you look like the bad guy. I'd get a third party to talk to him. A buddy or whatever. He's not going to hear this from you. And if he really does continue to prioritize this old friendship over his new marriage -aside from occasionally saying that the ring means something-ACTIONS NOT WORDS-then you need to let him go because to be honest, this never feels OK. It only gets worse. There will always be new women in the picture and he'll be used to the secret friendships.
"Honey I realize I've forced your hand here by making you choose between me and your old friend. I understand why your friendship means more since it's so...old. I understand you shouldn't HAVE to quit talking to her just because I'm uncomfortable with her slutty personality. But now she's your secret friend. Which I totally get. But I don't have to accept it any more than you have to accept my feelings. So. I'm thinking we're not on the same page in this relationship and going forward this will be a serious problem that will only get worse. Do you want to try counseling or anything to see what we can both come up with, or should we just call it quits. Cuz for real. i'm not living like this."
But you have to mean it.
Good luck.