YOU SAID:

I was riding a horse while texting to my friend.

INTO JAPANESE

私は私の友人に携帯メールしながら馬を乗っていた。

BACK INTO ENGLISH

I was riding a horse while texting my friends.

INTO JAPANESE

私は私の友人、携帯メールしながら馬に乗っていた。

BACK INTO ENGLISH

I was on a horse while texting my friends.

INTO JAPANESE

私は私の友人携帯メールしながら馬に乗っていた。

BACK INTO ENGLISH

I was on a horse while texting my friends.