The animal rights campaigner was working undercover investigating reports that the bus stopped at a farm selling medicinal bear bile that was extracted from Asiatic black bears crammed into tiny cages.
What she saw would stay with her forever - as she slipped away from the farmers selling "fresh bear bile" to passengers she found cage after cage crammed with pitiful bears unable to stand or turn around.
When one of the distressed bears reached out through the cage bars, Jill instinctively reached out too, taking the soft paw in her hands - and she knew she had to do something to help them.
Since then Jill has worked tirelessly to end the barbaric bear bile industry across China and Vietnam, while rescuing hundreds of bears from farms like the one she first visited.
It is estimated that more than 11,000 Asiatic black bears - better known as moon bears - are victims of the trade in the two countries, held in tiny cages and subjected to unimaginable daily suffering as their bile is taken from their gall bladders via a needle while they are bound and writhing in agony.
The bile is used in traditional medicine, cosmetics and wine, with many believing drinking the liquid boosts fertility.
Some bears may be kept caged for up to 30 years in a horrific industry worth over £1.5billion annually.
Jill, who lived in China for years, set up Animals Asia in 1998 to fight the cruel practice, with the motto "Until the cruelty ends", creating two bear sanctuaries in China and Vietnam spanning more than 180,000 square metres.
Her lobbying has helped to close 43 Chinese bear farms and make 20 of the country's 31 provinces free of bear farms.
She also made a breakthrough discovery that bear bile could be replaced with cheaper and equally effective herbs and synthetics - and began working with traditional medicine doctors and Western doctors in China and globally to promote the alternatives.
And in 2015, the organisation received an historic agreement from the Vietnamese Traditional Medicine Association to stop all use of bear bile by 2020 - raising hopes the horrific practice could be stamped out within years.
Jill, originally from Newark, Notts, says: "I really think we are making progress.
"Now, even the traditional users of bear bile are stating 'enough'. We are seeing farms close, bear numbers drop, and together we are making an unequivocal public commitment. The will to eradicate bear bile farming from Vietnam has never been higher."
"But the clock is running against us."
In Vietnam, extracting bile is technically banned, but it is still easy to find traders selling bear bile for as little as £1.50 per vial.
Jill says: “We have a good relationship with the Vietnamese government but they turn a blind eye to bile-bear farming.
"Men drink bile in rice wine, thinking it makes them virile or will stop hangovers. But it is not all just superstition.
"The bile does have beneficial properties. It can treat liver ailments, it reduces swelling when mixed with alcohol and rubbed on bruises, and rejuvenates brain cells, which has huge promise for people with Parkinson’s.
“But it has also been replicated successfully in a lab, so there is no need for a single bear to be in captivity and to suffer for decades until they die in the cages which have imprisoned them.”
At her sanctuary, which costs more than £1million a year to run, rescued bears, who often arrive in a terrible condition, are free to roam huge compounds.
The bears are rescued in a variety of ways, including being saved by the police from smugglers, who transport them to China where they fetch a much higher price. Others are bought from individuals or given to Jill and her team by bear farmers going out of business.
Jill says: “What is important is that we are on their conscience radar now.
"They now have an alternative to killing the bears if they no longer produce the bile in the quantities they want or if they are getting out of the business.
"And it really is a filthy business. In Vietnam, they often knock out the bears with the drug ketamine, which is itself illegal.
“If the bears are not knocked out they are splayed out and restrained while the needle goes in. You only have to think of the pain you would feel if a huge needle was inserted in you.
“Getting people to realise the benefits of synthetic bile will be another milestone in the eradication of the bear bile farms. We will achieve that – however long it takes.”
Film on sale 24th April, 2017 on DVD (amazon.co.uk for Europe and animalsasia.org for the rest of the world) and a download from iTunes and Amazon (US, UK, Canada and Ireland)