The rugby player is said to have been sexting a string of other women – even as Kelly was caring for him after he sustained a nasty facial injury in a collision with a bus.
Sales manager Stacey Simcox told The Sun how Danny had been texting her raunchy messages since they met about eight months ago.
"He'd send me filthy messages about what he wanted to do to me and begging to meet up for sex," she said.
"It got progressively more explicit and I just thought, 'What a rat'. He's going out with one of the most stunning and successful women in Britain."
Stacey added: "I just couldn’t understand why he'd stray. Every man fancies Kelly, but that obviously wasn't enough for Danny."
Kelly reportedly found the texts and kicked him out of her home last weekend after a blazing row.
She has since deleted pictures of the couple looking happy and relaxed on holiday from her Twitter and Instagram accounts.
The on-off pair dated for a couple of years from 2008, and then got back together when Kelly’s romance with his England teammate Thom Evans hit the rocks.
On Sunday she posted a cryptic message on her Instagram.
"Some people come into your life as blessings. Others come into your life as lessons," the quote states.
Kelly may have had some prior warning about Danny's behaviour, as Katie Price recently boasted that she had evidence the rugby star was cheating.
The glamour model and businesswoman had been feuding with Kelly over Twitter and implied Danny had been less than complimentary about his girlfriend in secret texts.