Wallace denies charges of murder and throwing a corrosive fluid with the intention to burn, maim, disfigure, disable or commit GBH.
Prosecutors claim Wallace carried out the attack out of jealousy after van Dongen began seeing another woman, Violet Farquharson.
Farquharson on Tuesday told Bristol Crown Court she met van Dongen via a dating website around the end of July, 2015.
She told the jury: “I did see injuries on him. He had gashes on him, a gash around his neck and gashes on his torso as well.
“He said she attacked him. He was quite off the cuff about it.”
Farquharson said when van Dongen was at her flat they received 14 silent calls and he ended up buying a new phone.
She said there came a time when van Dongen promised her he would finally tell Wallace it was over between them, and he stayed out overnight on September 22.
In the early hours of the next day police told her van Dongen had been taken to hospital.
Farquharson told the jury: “They (police) took me to hospital to see him. He was in theatre. He was crying out in pain. He said he wanted to die.”
Van Dongen remained at Southmead Hospital for the next 14 months, with Farquharson visiting him almost every day.
She recounted how van Dongen was then transferred to a Gloucester care home but was taken to a Belgium hospital where she visited him the week before Christmas.
Though he was very depressed it was a pleasant visit, she said, and they talked about spending Christmas 2017 together.
But after she returned to Bristol she was called by van Dongen’s brother, who told her Mark had died on January 2 this year.
Shaun Groves, a criminal intelligence analyst, said police found two photos of Farquharson on Wallace’s iPhone.
Analysis of her laptop computer revealed an Amazon purchase record for sulphuric acid on September 2, 2015, as well as later web searches such as “What happens when you swallow sulphuric acid” and “Can I die from drinking sulphuric acid.”
There was also a record of sulphuric acid-related court stories online.
The case continues.