Cowell tweeted a link to the girl's parents' Change.org petition. Asking his 12.1 million followers to help "save this wonderful girl". He included the hashtag #SaveAbi.
The Longfellow's launched the petition to ask David Cameron to help them get the medication Abi needs on the NHS.
The petition is nearing its target of 200,000 signatures and Cowell's tweet is sure to give it huge exposure - especially considering his tweet received more than 1,000 favourites and more than 1,000 retweets, too.
Abi's parents were overjoyed at the support they have received since launching their petition.
Mr Longfellow said on ITV's Good Morning Britain: "The help that the nation has given us is just fantastic, the support, the messages, it's phenomenal to be honest with you."
Mrs Longfellow added she had been "astonished" at the number of people that have got in touch and spread the word.
Brian May also shared a link to the petition on his Facebook page and wrote: "Save this little girl."
It prompted hundreds of comments from his 521,000 fans on Facebook to say "signed".
Abi's parents say they were told the disease their daughter has is too rare to justify the cost of the drug.
They explained that the manufacturer - Alexion Pharmaceuticals - has refused to drop the £136,000-a-year price, which means it is impossible for them to buy it.
"That’s why we’ve started a campaign with the Sunday People," her parents wrote on the petition page"
"Abi’s one-in-a-million illness is too rare to be covered by NHS England’s drug funding policy but not rare enough to unlock cash for exceptional cases.
"Knowing that the drug sits in the hospital but Abi can’t have it is horrendous."
Mrs Longfellow said the drug is used in other countries but not in the UK, so they are calling on the manufacturer and the Government for help.
As part of the appeal Abi also penned a moving letter to David Cameron, in which she wrote:
"Why is that fair when other children with other rare kidney diseases like me are receiving it?
"I’m a 12-year-old English girl and if I lived in Ireland, Europe, America, Canada etc. I would get the medicine without hesitation."
To sign the petition, visit Abi Longfellow's page on Change.org.