I saw my friend laughing his head off and it made me laugh too.
My friend was killing himself laughing and it got to me too.
ーI saw my friend laughing his head off and it made me laugh too.
「友達が大笑いしているのを見て私もつられて笑った」
to laugh one's head off で「大笑いする」
to make me laugh too で「私もつられて笑う」
ーMy friend was killing himself laughing and it got to me too.
「友達が大笑いしていて私にもうつった」
to kill oneself laughing で「大笑いする」とも言えます。
to get to me too で「私にも（笑うのが）うつる」＝「つられて笑う」とも表現できます。
ご参考まで！
"I saw my friend laughing so hard it made me laugh."
"My friend was laughing so hard I couldn't help but laugh as well"
- "I saw my friend laughing so hard it made me laugh."
"I saw ~" 「〜を見て」
"my friend" 「私の友達」
"laughing so hard" 「大笑いしている」
"it made me laugh" 「ついつれて笑った」
- "My friend was laughing so hard I couldn't help but laugh as well"
"I couldn't help but laugh as well" 「私も思わず笑ってしまいました」
カナダ 
オーストラリア 
