she <wet her hair> or she <wetted her hair>?

According to Professor Pam Peters, the author of “The Cambridge Guide to English Usage”: “wetted” is used for the past tense when there’s a reference to a deliberate action.

So, I guess it should be “she wetted her hair” and not “she wet her hair.”

Here is what she says in her book:

The choice between wetted and wet for past participle again helps to show whether it’s the product of human intervention, or a more or less natural result:

……..his straight brown hair, freshly wetted and parted in the middle .

The wall had been wet by a broken pipe for many years.

He wetted his lips in a theatrical way.

"She wet her hair" is perfectly grammatically sound for where I live. The only context where I can think the distinction is really going to cause any confusion or lead to anyone even thinking about it is if you're talking about having wet yourself vs having been wetted, although I don't know that the second would ever come up in context. It's probably a regional thing, if that's what a modern book says, but even outside the US I don't think most folks take issue with "she wet her hair" instead of "she wetted her hair".