Rule for using "for" vs. "to"

One possibility, understanding that prepositions have very slippery and often idiosyncratic meanings ... have her think of to as indicating a destination:

I sent the present to him.

I gave the present to her.

Whereas for can indicate or "for the good of":

I did it for her.

The present is for him.

Do you think this might help? Note that we're likely to find many cases where these simplistic definitions don't work, alas.

The problem is that both "for" and "to" translate to Portuguese in these cases as para.:

"The present is for Thomas." --> O presente é para o Thomas.

"Say hello to your wife." --> "Diga oi para a sua esposa."

As a native Portuguese speaker (I'm Brazilian too), I'd say that there's no simple rule of thumb to always avoid this confusion. You can explain to her what Mike Pope said in his answer; that should help. But it's only by listening and repeating the appropriate usage that we will naturally learn to use these prepositions correctly.

This is my advice for her... I mean, my advice to her.

I'm a spanish speaker and I sometimes confuse them too, because we use the word "para" with both meanings.

In this example "Say hello to your wife" and in all the sentences where you have "dative case" you should use "to" I think.