It is a good idea for your Witch to actually learn some spellcasting
The real problem: the Witch player underestimates spellcasting
From your comments I have understood that your Witch player doesn't believe in Witch's spells actually being strong, thus doesn't consider investing time and effort in learning to use them. However, Witch is considered a Tier 1 spellcaster, perfectly fitting the definition of Tier 1: she is, just like Wizard and Cleric, a prepared spellcaster, which means that while it takes time to learn the class, with right application of her abilities she can literally win the game alone.
Once your Witch player learns to utilize the class spells, even a bit, it will be fun and the feeling of having a wasted (unused) feature will go away -- just as the feeling of wasting time on learning something weak.
Witch is actually hard to learn
Just as other T1 spellcasters, the Witch is a lot of content to discover, and it will be really hard for your Witch player to do it all on her own experience.
Pathfinder RPG is full of "traps" -- crap options that sound cool but actually aren't cool, and vice versa, and of unexpected synergies of some seemingly unrelated abilities. Witch has a lot of useless spells, but also a lot of powerful ones, and for a newbie it may be kind of hard to distinguish one from another.
You say that the Witch was MVP just on her Hexes: it means that your group doesn't use any crazy optimization schemes. It turns out to be both an advantage (not much to learn to feel the spells being more useful) and a disadvantage (game is too forgiving, which makes building own experience harder).
What is much more important: it can be very easy for the Witch player to overdo and become too strong for your current group. She will just overshadow everyone and other players will have nothing to do.
Witch handbooks: her way to learn the right spell usage
Handbooks are fan-made comprehensive guides to a class. They are written by Pathfinder veterans with a lot of experience and proofread by a huge playerbase of other veterans, so knowledge from handbooks is actually very useful.
You need one of the Witch Handbooks, for example, this one. I do not say that it is good or bad (I didn't read it myself), but seems to be pretty popular, and also has links to other guides: all of them refer to each other and say that all contain useful info.
In this guide your friend can get some basic advice regarding Witch spells, how do they function, when is it a good idea to use a spell and when is it better to do something else. If she doesn't have (doesn't find) time to use them during combat, she can prepare some spells for non-combat usage (aforementioned healing the group by CLW). The handbook will give her the understanding which spells are OP, which are good, and which are crap to be better avoided. Just prepare something useful everyday and don't forget to experiment: the Witch is a prepared spellcaster, which allows some daily flexibility in choosing spells.
After some time of "cooking by the book", if she actually tries it, she will start to understand things herself and likely find spellcasting fun. For example, if after every combat she looks at her list of prepared spells and asks herself "What could I use now", even if she wastes some spell slots, her experience will already come fast, she will see the spells she can use and spells she can't use yet.
You can help her too
If she is too lazy to read the handbook, you can read it for her: by the sole fact that you came here to ask a question and cooperated with me to find the real problem I assume that you have the required patience and energy to read a bit. If you have other spellcasters in your group, they can possibly attempt to share some experience too. Just don't make it to a level when she doesn't feel like playing her char herself anymore... Don't teach her during your sessions -- unless you all want to do it in-character and ready for that kind of RP.
Beware of her power!
Again... Be ready, though, that if she starts to use her spells while already being MVP, other members of your group would probably have to also adjust their optimization level a bit. Or more than a bit.
Note that given your low general optimization level, she wouldn't have to learn too much to unintendedly make some players feel useless.
I wish you all good luck in your adventures.
Appendix: useful links
Here you can find all interesting pages about the Witch class that I found.