“My mother-in-law treats my husband like he's still her little boy”

What could be going on

Enmeshment: Fits if their closeness leaves no room for his separate adult life.

Separation-individuation (Mahler): Could be unfinished separation on both sides; check whether he resists it.

Jealousy: Check whether she acts this way most when you are present.

Oedipal dynamics: Could be she sees you as a rival for his attention.

Differentiation of self (Bowen): Check whether your husband can hold his own position with her.

Also said as

• his mum still does his laundry and calls him baby
• my husband's mother acts like his wife
• mother-in-law too involved in my partner's life
• his mum can't let him grow up
• mother-in-law babies my husband
• my husband is a mummy's boy