How to prove formally that grammar isn't LR(1)
$$S'\Rightarrow^*\underbrace{ab}_uA\underbrace{b}_w\Rightarrow \underbrace{ab}_u\underbrace{b}_v\underbrace{b}_w$$

$$S'\Rightarrow^*\underbrace{abbbb}_zA\underbrace{b}_x\Rightarrow \underbrace{ab}_u\underbrace{b}_v\underbrace{bbbb}_y$$

$$FIRST(w)=FIRST(y)=b$$
But:
$$abAbbbb=uAy\neq zBx=abbbbAb$$