The former Olympian commented "cutie" underneath her son's selfie, which bizarrely prompted the troll to reply "you comment on every post to get attention".
Jett wasted no time in calling out the woman, hitting back: "Who the f**k are you?"
Jett also publicly named and shamed the troll on his Instagram Story, where he posted a screenshot of their exchange.
"Firstly, they don't even follow me so they've gone out of their way to be annoying," he wrote.
"Secondly, I wouldn't normally do this but you've got me at the start of a new mindset so congratulations on your shoutout. Thirdly, grow up idiot."
The troll's comment was all the more cruel given just a day earlier, Lisa said she was struggling to cope with the death of her daughter Jaimi, who passed away last year.
The retired professional swimmer said she "couldn't get through each day" without the support of her husband Mark Tabone.
"Thank you to my hubby… for everything. He's so caring and supportive. I couldn't get through each day without him," Lisa wrote on Instagram.
Lisa's oldest child Jaimi was 33 when she passed away following a private battle with a long-term illness last September.
In July, Lisa led the tributes to Jaimi on what would have been her 34th birthday with a heartbreakingly beautiful Instagram post.
"Our beautiful Jaimi, born at 9.03am, 34 years ago," the mother of three began.
"Today is a hard day. I can't even explain how I feel. I have no words."
"Happy birthday Jaimi. We all miss you and love you greatly.❤️😍😞🌸👼 #rememberingjaimi," Lisa signed off her poignant tribute.
Since losing her beloved daughter Jaimi in September last year, Lisa has been open and honest about her experience with grief.
Most recently the former Olympic swimmer paid tribute to her late daughter by creating a special memorial garden.
"4 months ago, the doctor gave us the news we had dreaded for years, that Jaimi would not survive the night. It was like they just ripped our hearts out. Total disbelief... and even though we had prepared ourselves for the worst for weeks, months and even years and thought we were prepared... nothing... absolutely nothing prepares you for that moment or those words," Lisa revealed at the start of January of the harrowing moment she lost her oldest child.