The saleswoman Lucy is her mother's best friend and the two have been inseparable since the teenager's parents split up six and a half years ago.
To cheer up her teenage daughter, mom Rachel, 48, would treat her to McDonald's and other junk foods to "keep her happy".
This has inspired the love of food in teenagers who she admits has become a problem.
Lucy of Lightwater, Surrey said, "If I could marry pasta, I'd rather marry pasta than someone else. Some people find comfort in a book, but I find it in pasta."
The couple share an apartment and mother Rachel always makes sure that her daughter gets what she wants when it comes to food.
Between them, they eat a massive 10,000 calories every day.
Lucy's favorite food is creamy carbonara noodles, which she likes to mock up with a huge slice of garlic pizza bread.
Rachel explains, "When I first broke up, I'd treat her with McD's just to cheer her up, and it's a habit I can't give up."
And even going to work has problems for Lucy because she is surrounded by fast food and sweet treats that she eats all day.
She said, "If it's a multi-pack, I'll eat the whole thing."
One of the teenagers favorite spots for snacks is her bedroom, and she admits that "sometimes at night I turn around with a food wrap stuck to me in bed".
Rachel works the Royal Mail at night, so Lucy is often home alone ordering an unhealthy takeaway for her dinner.
She said, "I eat out of sheer boredom."
But Lucy's increasing weight has had a devastating effect on her confidence.
She said, "I literally look pregnant, I just feel absolutely huge. My weight makes me feel like I'm crying. It hits a point in my heart and I'm locked in a box.
"I feel like everyone thinks I'm gross and I think I'm gross."
Lucy has piled three and a half stones in the past two years – and there's another problem, her love life.
She said, "Four years ago I came out to see my mom. She was fine, I was so naughty. I'll see girls on dating apps and I think I'm not thin or pretty enough. That scares me that I'll be alone forever
"If I were thinner, I would enjoy life to the fullest and go clubbing with my friends. Something has to change, something has to break the cycle."
"I feel like mom always says yes to make me happy, which has led to my weight gain."
So the 18-year-old decides to enlist her mother's help and both go on a weight loss journey together.
Lucy cannot put into words how she is feeling and writes Rachel an emotional letter.
She says, "My weight makes me feel like I have no self-worth. I feel so ugly. We both have to do something about our weight. Let's say yes to self-love and begin this journey together."
Devastated, Rachel vows to help her daughter get healthier.
She said, "I think I have to be more parents than friends. I should have said something the first time I saw the weight, but I didn't mean to piss you off so I didn't say anything."
"I used food to comfort her, which I shouldn't have done."
The mother and daughter duo turns to personal trainer Raymond for help, and he has some shocking numbers for Lucy and Rachel.
He weighs the couple with Rachel, who tips the scales at 17stone, and a BMI of 43.1. Daughter Lucy weighs 15stone 5lbs and has a BMI of 38.9.
This puts them both in the "red zone," which means they are at high risk of developing heart disease, high blood pressure and diabetes.
Raymond has come up with an extreme fitness program for Sex Week – but right after her first session, Lucy scoffs at a bag of mini cheddars in the car with her mother on the way home.
Determined to do whatever it takes to keep Lucy and Rachel on track as they eat, however, Raymond goes to the family home to clear their shelves with goodies.
They fill six trash bags with junk food and sugary treats, all of which will be donated to a food bank.
Raymond leaves the couple a list of recipes that include three healthy meals a day plus three healthy snacks.
On the first day of her new healthy eating regime, Rachel works a night shift at the Royal Mail and instead of a high-calorie snack, Lucy has to make her own low-fat stir-fries.
Surprisingly, she loves it and "can't believe it's healthy".
(Image: Twofour Broadcast. All rights reserved.)
After her first week, Lucy has lost a staggering seven pounds – but she's indulging in a giant nandos.
Rachel then cooks her daughter a steak dinner with fries as a "treat," but realizing that her gentle approach to discipline is not helping her daughter, she enlists Raymond's help in handing out a hard love.
He takes her first thing in the morning in the rain to run through the park.
In the final week of her six-week intense weight loss journey – and right at the start of the coronavirus lockdown – Lucy developed flu-like symptoms and had to self-isolate at home.
Raymond checks in virtually and encourages her to stick to the plan. Though both mother and daughter can't go out, Rachel tries to keep her pace up by walking around the apartment.
And there's good news for Lucy, she lost a different size and is now back in her size 14 jeans.
At the last weigh-in, Rachel, who weighed a massive 17-stone, lost a whopping 12 pounds and is now only 16-stone-2-pounds. Also, she has lost 14 cm from her waist.
And for Lucy, who weighed 15 pounds 5 pounds, she managed to lose 5 pounds and is devastated – but Raymond insists he is proud of her for losing some weight.
She also managed to lose five inches from her waist.
And five weeks after the last weigh-in, Lucy is sticking to her healthy diet and is happy when her body continues to shrink.
- It's your fault I'm fat, it's 9pm on 5Star tonight.