tell her
tell 'er
/ˈtɛlɚ/ElisionLinking
In real speech
What happens to the sounds
The h in unstressed her, him and his disappears, and the pronoun sticks to the previous word: tell her → “TELL-er”.
When to use it
Same with love 'im, ask 'er. Normal in any conversation; the h comes back when the pronoun is stressed (tell HER, not him).
Casual → formal
Slang
tell 'er
Neutral
tell her
Formal
inform her
Sound notes
Elision: a sound disappears completely (often t, v, h or th).