The vet said she might have kidney failure if I didn't do anything about her condition.
The vet told me she might suffer from kidney failure if she didn't get any treatment.
Summerさんの英文は次のように言うとより自然になりますよ!
ーThe vet said she might have kidney failure if I didn't do anything about her condition.
または次のようにも言えます。
ーThe vet told me she might suffer from kidney failure if she didn't get any treatment.
「獣医は彼女が治療を受けないと腎不全を患うかもしれないと言った」
ご参考まで!