Jennifer began dating the famed bad boy in 2008, but the pair went their separate ways a year later.
Katy and John got together in July, following the Teenage Dream singer’s split from comedian Russell Brand.
“Jennifer gave Katy her number and told her to call any time, but she didn’t expect her to take the offer so seriously,” a source revealed to British magazine Closer.
“Now Katy’s in contact and asking her for tactics on how to tame him, but John seems to know about it and isn’t happy that his ex has become so involved in his relationship. Jennifer’s trying to wean Katy off, telling her that she doesn’t want John to think she’s meddling, but Katy won’t be put off.”
Katy has a complicated relationship history herself, having fallen for bad boy Russell in the past.
Now it seems the same thing has happened with John, as the pair’s relationship gets more serious.
“Katy is besotted with John,” an insider said.
“When they first started dating, she swore she wouldn’t fall for his womanising ways, but now she can’t get enough of him and has been telling friends that he’s not the player that people believe he is. But her friends aren’t convinced and are worried that she’s just falling for another bad boy.”