Se or Ses?

Question

In Lesson2.3 There is a question test:

"José les tira las serpentinas a ellos."

and the answer is:

"José se las tira."

But the indirect object pronoun is "ellos"

"les" should change to "ses".

Why does "le" change to "se"?

Answer 1

Read over this:

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Answer 2

Thank you.