There are two issues here, I think.
One is that your mum is letting him have chocolate and keeping it from you
Two is that your partner seems very controlling. You were "expected" to eat "clean" when you were pregnant(!)
I would be furious if mum my was undermining me in this way, yes, but it's only a little chocolate not alcohol (for perspective) and I wouldn't throw away the relationship for it.
What you've said about your partner makes me see her actions in a different light too.
No one has the right to enforce a style of eating on any adult as he has done you. You say he's never liked your parents and doesn't want you to see them, even before this kicked off. These are major red flags and he's ticking some key boxes of coercive abuse.
In light of that, I wonder if your mum is probably trying to counterbalance the control he exerts over you and your children and whether it's misguided or not I would forgive her.
Under no circumstances would I allow a rift to develop with your family here. For your own sake.
Please read up on coercive control and other forms of abuse:
Some common examples of coercive behaviour are:
- Isolating you from friends and family
- depriving you of basic needs eg food
- monitoring your time
- monitoring you via online communication tools or skyward
- taking control of aspects of your daily life such as what you wear, where you go, who you can see, when you can sleep and what you can eat
- depriving you of access to support services eg medical help
- repeatedly putting you down eg telling you that you are worthless
- controlling your finances
- humiliating you, degrading you, dehumanising you
- making threats and intimating you
Other common forms of coercive control include emotional manipulation such as:
- you wouldn't do this / you would do that if you loved me
- claims that your actions or attempts to establish boundaries result in them feeling suicidal or threatening self harm (they may act out some of this)