A ella puedes contárselo todo

[Premium]

You can tell her everything.?

[AngelicaDeAlquezar]

Yes.
When you have a combination of indirect and direct object pronouns, "le" turns into "se".

[abuelito]

Yes.
When you have a combination of indirect and direct object pronouns, "le" turns into "se".

Just an addition to your quote: :) "le" or "les" turns into "se" before a direct object pronoun that begins with "l"