Como puedo yo ayudarla? means How can I help you? But why it is not used “Como puedo yo ayudarle?” Because in another sentence he said; No se por que el no le escribe. I thought that “le” is used for “to you” in formal (usted)
Thanks for helps.
It must be because the person is referring to a woman como puedo yo ayudarla (How can I help her).
Edit: I see the example is from who is she I did at one point study it and I am somewhat familiar with the story. The story has something to do with a woman looking for her brother. It is best that you wait for a native speaker to explain further. Looking at it more closely I am not so sure about the difference between the two phrases, especially the second one.
This is a bit complicated. Let me explain.
In principle: “le” is indirect object, it can be used to refer to third person singular both masculine or feminine, or to second person formal (usted)
Le doy un regalo: I give a present (to him, to her, or to you (formal ) - German “ihm”, "ihr, “Ihnen” (singular)
On the other hand, “lo” and “la” are used for direct object (both third person and second person formal), “lo” is the masculine form, “la” is feminine - German “ihn”, “sie”, “Sie” (singular)
Lo he visto: I saw him, I saw you (formal, masculine)
La he visto: I saw her, I saw you (formal, feminine)
However depending on speaker and region, there may be some mixing up of forms. The reason is that Spanish uses an “a” to mean direct object when referring to persons. That sometimes makes speakers hesitate about what pronoun to use and you get several variants.
This changes depending on region, as I said, you may want to google “leísmo” and “laísmo” for more information on this but the main takeaway is that you may find hesitation in some cases.
For example it’s true that some speakers sometimes feel that they should use “le” to mean “usted” even in situations where “lo” or “la” would be used for third person. That’s especially the case in some parts of Spain, not so much in Latin America, it’s mostly unconscious and not very consistent. Anyway it’s not considered standard.
Your example is particulary interesting. If you ask a native speaker of Spanish without a lot of interest in grammar whether “ayudar” should be used with an indirect object or direct object, you may find h/she may be at a loss.
The reason is the following:
We always say “ayudo a mi amigo/a” we have an “a” but is it because it’s indirect object? Or because it’s a personal direct object? No idea! Notice that “logically” you can have both kinds of object, as you can tell by comparing languages:
- In German it would be an indirect object: ich helfe dir (etwas zu machen)
- In French/Italian, it’s a direct object: Je l’aide (à faire quelque chose), lo/la aiuto a fare qualcosa
So, for many Spanish speakers this is not clear and you find hesitations. They really don’t care much about the difference, so you can hear “ayudarlo”, “ayudarla” or “ayudarle”. Some contexts tend to produce one of the variants more often than the other. For example, for some speakers when referring to “Ud”, the “le” variant may sound better, but, again, that changes and it’s not at all official or standard.
At any rate, there’s no difference in meaning.
If you ask me (who consider myself a more grammar-conscious speaker than the average) what the “correct” variation is, I’d say it’s “ayudarlo”/“ayudarla”, never mind if you’re referring to a third person or to second person formal
However, I’m sure many people around here would find that puzzling
Bottomline: don’t sweat the detalis, get used to some hesitation in that domain
In case you’re very interested, here is a thorough explanation, partlly in German. I think you understand it well:
http://www.hispanoteca.eu/Foro-preguntas/ARCHIVO-Foro/Laísmo-leísmo-loísmo.htm
Anyway, your examples would be both gramatically correct (following the standard “Royal Academy” rules):
No sé por qué no le escribe (because it’s indirect object) doesn’t write to you
¿Cómo puedo ayudarla? direct object: How can I help you
Again, the standard rule is that you use the exact same pronouns for second person singular formal (usted) as you’d use for third person singular (él/ella), that is, “le” when it’s an indirect object, same for both genders, “lo”/“la” when it’s a direct object, depending on gender
But, again, expect variations
You, sire, are a legend. 
Thanks for your help, mate. Sometimes I am confusing, but you have explained it very well. Cheers.