Cullen currently trains Armstrong, who races for Chip Ganassi in the US IndyCar series, having previously been physio to Formula 1 star Lewis Hamilton.
The pair first teamed up in 2016 before ending their professional relationship in 2023, with the Kiwi a key factor in Hamilton's success during that time as he added four titles to his world championship haul.
Since teaming up with Armstrong in March of this year, Cullen has been an influential figure in her compatriot's upturn in form, playing an instrumental role in the 23-year-old delivering two impressive fifth-placed finishes.
Their partnership also appears to be blossoming off-track ,with the trainer often posting pictures on social media, while recent reports indicated the pair are now living together.
Armstrong was in action at the historic Indy 500 on Sunday, but suffered an engine failure which caused him to retire from the race after just six laps.
Posting on her Instagram account, Cullen vowed that the team will 'be back', adding that they will learn from the heartbreaking experience.
The post read: "We came, we learnt, we grew, we all pushed so hard.
"So proud of every one of you and the incredible hard work you all put in over these crazy two weeks...