But there were red flags in his obsessive digital surveillance of her location, which was seen by his friends as an indication of how much he liked 21-year-old Lilie James.
Another warning sign emerged when Thijssen showed an intimate image of Ms James to his friends without her consent - a move that experts say was calculated to assert his dominance.
Neither display of technology-facilitated abuse raised concerns for his friends, which experts believe is a testament to how worryingly normalised this behaviour has become among young people.
Thijssen's pattern of controlling and coercive behaviour ended on October 25, 2023 when he barged into a bathroom where Ms James was changing at St Andrew's Cathedral School in Sydney.
The 24-year-old used a hammer to fatally strike her at least 25 times in the head and neck after practising the murder earlier in the day during several "dry runs" captured on surveillance cameras.
Following an inquest into her death, State Coroner Theresa O'Sullivan on Thursday found Ms James had been killed by her ex-boyfriend in the context of domestic violence.
She identified a number of critical actions that can be taken to address technology-facilitated abuse through education, awareness raising and bystander intervention initiatives.
Domestic violence expert Kate Fitz-Gibbon, who gave evidence at the inquest, welcomed the findings.
"In the midst of a national crisis of men's violence against women and children, these recommendations should be progressed with urgency and supported with appropriate funding," she said.
The coroner adopted Professor Fitz-Gibbon's suggestion that domestic violence screening tools should reflect the link between coercive control - including technology-facilitated abuse and physical stalking - and the risk of intimate partner homicide.
"Attention should be paid to this recommendation in all Australian states and territories," Prof Fitz-Gibbon said.
"These abusive behaviours can indicate a risk of escalation of violence, and intimate partner homicide – and should not go undetected."
Stalking - including online - occurred before 41.5 per cent of intimate partner homicides in Australia, data from the Domestic and Family Violence Death Review Network shows.
But more than one in 10 people think it's reasonable to expect to track a partner during relationships, according to findings from the Social Research Centre.
It shows that the younger generation of digital natives have normalised abusive behaviours online like harassment and coercive control, according to educational organisation Teach Us Consent.
"We must take abuse online as seriously as we do abuse in-person," chief executive Chanel Contos said.
"Young people deserve the culturally relevant education about technology-facilitated abuse they need to form healthy relationships."
Ms O'Sullivan echoed the sentiment when she endorsed the development of respectful relationship advice services, particularly for young men aged between 19 and 24.
1800 RESPECT (1800 737 732)
Lifeline 13 11 14
Men's Referral Service 1300 766 491
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