Several people in my group has been discussing whether it was possible for a witch to learn spells from a scroll or a wizard and we can't find an definitive answer.
The rules for witches learning spells are here. It does include rules for learning from a scroll, much like a wizard, but the witch’s familiar cannot learn directly from a wizard—instead it must learn from another witch’s familiar, which a wizard cannot do. So a witch and wizard have the inefficiency of needing to create scrolls as a shared medium when trying to teach one another spells.
Personally, I’ve always just hand-waved that issue, though.
In all cases, the witch can only learn those spells that are on the witch spell list.