Naomi Elliott and Lois Summers from Harefield are taking on a 15,000ft skydive to help Jess Shepherd, 8, from Ruislip. It was due to take place today but bad weather has delayed it until 25 September.
Jess is suffering from a rare form of childhood cancer for the fourth time.
See all of the coverage on Jess's campaign here.
The money raised through the event will go towards the £300,000 goal to help the Harefield Primary School pupil to get to America for special treatment she can't get in the UK.
Danielle White, Jess's mother, has called the support from the community as “ridiculously overwhelming” and praised her friends of ten years for conquering their fear of heights.
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Pictured: Sisters Naomi and Lois, who are taking on the challenge
Speaking about what made her and her sister willing to face their fear of heights in the name of charity, Lois said: “Jess is conquering something massive, you know, and what other way than to jump out of a plane at two and a half miles up – and be terrified!
“We’ve got kids of our own and I would jump out a plane without a parachute for one of my kids.”
Lois and Naomi have put £1,500 towards the fund for the sky dive and are hoping to raise more through sponsorships and raffle ticket sales on the day.
Danielle said: “She’s absolutely loving the idea. She’s very close friends with Naomi’s daughter, Mia, and they’ve got a little friendship going.
“They’re enjoying the fact that their auntie and their mum are going to be throwing themselves out of a plane. She thinks it’s pretty cool, she can’t wait to watch.”
Danielle described her pals' efforts as “absolutely mental”.
She said: “I personally share their fear of heights so I can’t imagine what they’re going through. I cried on the London Eye!
“It's overwhelming, the response Jess gets in general. This is probably one of the most extreme things anybody so far has gone through, to help and support Jess along the way and raise money.
“It is overwhelming that the two girls have decided to do it. It’s a lovely idea and they’re doing it for the right reasons.”
Jess named her tumour Boris and thereafter the Help Jess Beat Boris campaign sprung into action. She recently met Uxbridge and South Ruislip MP Boris Johnson, pictured below.
The sky dive will be taking place at the Old Sarum Airfield in Salisbury.
Another event has been raised to add to the cause. A music talent show takes place on October 8. See details below.
The Harefield community has come together for a number of events to raise money for Jess, including an open air festival and a fun day at the Harefield Cricket Club.
The family have also seen support from Ruislip in the form of auctions and raffle tickets.