I feel horrible. my niece and nephew brought a stray home to my mom's house and it was just covered…

I feel horrible. my niece and nephew brought a stray home to my mom's house and it was just covered in ticks and was really bloated (a sign of worms) and had diarrhea. my mom and I took it to the shelter and I know my niece is gonna be devastated but none of us can afford to get it the help it needs. but the people at the shelter were really sweet and said she'd get adopted pretty quick. I don't know if we should just say it ran away or tell them the truth. help