Lambert's boyfriend, singer-songwriter Anderson East, took to Instagram on Thursday to criticize Brooks' performance at this week's Country Music Association Awards.
Brooks -- named CMA's entertainer of the year for a record sixth time -- caught flak from East for lip-syncing Wednesday night through a performance of his single "Ask Me How I Know."
"I keep a lot of my opinions to myself and respect anyone making music but as a person who tries to put on the best and most honest show I can night after night... this truly offends me," East wrote in a post that included a tweet about Brooks lip-syncing. "I was told country music is three chords and the truth."
East and Lambert went public with their relationship three months after she and ex-husband Blake Shelton announced their split in 2015.
Brooks explained backstage at the CMAs that he opted not to perform live as he's been touring heavily.
"We're in the middle of 12 shows in 10 days," Brooks said. "Not 10 shows in 12 days, 12 shows in 10 days. That band and crew works harder than anybody. The voice is gone."
After rehearsal for the CMAs the day of the awards show, Brooks said, he decided to lip-sync.
"We did a game-time call on whether to sing to track or lip-sync, and decided to lip-sync," Brooks said. "The voice just isn't there anymore, and you want to represent country music as best you can."
The singer had some kind words for his friend Lambert during his backstage chat.
"She's one of the few females that we play so you'd think she's want to play it safe," Brooks said of Lambert's throwback performance of "To Learn Her." "She came out and stuck country music in all our faces, traditional country music."