The song is from season 3 episode 13 of MLP:FiM.
♫ Lyrics for the song:
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[Twilight Sparkle]
A true, true friend helps a friend in need
A friend will be there to help them see
[Twilight and Fluttershy]
A true, true friend helps a friend in need
To see the light that shines from a true, true friend
Rainbow Dash: Um, hello! Friend trapped inside, remember?
[Twilight Sparkle]
Rarity needs your help
She's trying hard doing what she can
[Fluttershy]
Would you try, just give it a chance
You might find that you'll start to understand
[Twilight and Fluttershy]
A true, true friend helps a friend in need
A friend will be there to help you see
A true, true friend helps a friend in need
To see the light that shines from a true, true friend
Rainbow Dash: Uh, what just happened?
Twilight Sparkle: There's no time to explain, but we need your help. Applejack's trying to make dresses!
Rainbow Dash: Say no more!
[Rainbow Dash]
Applejack needs your help
She's trying hard doing what she can
Would you try, just give it a chance
You might find that you'll start to understand
[Twilight, Fluttershy, and Rainbow Dash]
A true, true friend helps a friend in need
A friend will be there to help them see
A true, true friend helps a friend in need
To see the light that shines from a true, true friend
Rarity: [gasps] Oh my, what a terrible dream I had. Or, maybe I'm still having it.
Twilight Sparkle: Rarity, Pinkie Pie is about to lose the apple farm. We need Applejack's help!
Rarity: Lose the apple farm? Well we can't let that happen, now can we?
[Rarity]
Pinkie Pie is in trouble
We need to get there by her side
We can try to do what we can now
For together we can be her guide
[Twilight, Fluttershy, Rainbow Dash, and Rarity]
A true, true friend helps a friend in need
A friend will be there to help them see
A true, true friend helps a friend in need
To see the light that shines from a true, true friend
[key up]
Applejack: Yee-haw! Now that's more like it, what's next?
Twilight Sparkle: The townspeople are furious, we need the old Pinkie Pie back.
Applejack: I'm on it, I know just the thing.
[Applejack]
The townspeople need you, they've been sad for a while
They march around, faces frown and never seem to smile
And if you feel like helping, we'd appreciate a loooooot
If you get up there and spread some cheer from here to Canterlooooooot!
Pinkie Pie: Come on ponies, I wanna see you smile!
Crowd: Pinkie!
[All and chorus]
A true, true friend helps a friend in need
A friend will be there to help them see
A true, true friend helps a friend in need
To see the light (to see the light)
That shines (that shines)
From a true, true friend!