The 38-year-old actor dated the chart-topping singer from 2010 until 2016 and after Demi nearly died of an opioid overdose in July, Wilmer has been a regular visitor to the rehab centre where she's being treated.
Demi, 26, has recently transferred from a rehab clinic on the east coast of the US to a world-renowned facility that specialises in tackling addictions, according to TMZ.
Wilmer has flown there several times in order to spend time with Demi and to support her through her struggles.
Wilmer and Demi have been spotted together at a local coffee shop, leading many of the locals to assume they're dating.
Although their relationship status remains unclear, Wilmer has been an almost constant presence in her life since Demi was rushed to hospital.
News of Wilmer's supportive presence follows Demi's mother, Dianna de la Garza, admitting she finds it ''really difficult'' to talk about her overdose.
She revealed that memories of her daughter's condition make her ''shake''.
Dianna said: ''It's still a really difficult thing to talk about. I literally start to shake a little bit when I start to remember what happened that day.
''[Demi's assistant called and] said, Demi overdosed. So, I was in shock. I didn't know what to say. It was just something that I never, ever expected to hear, as a parent, about any of my kids.
''[Her assistant] stopped for a second and said, she's conscious, but she's not talking.''
Dianna admitted to praying en route to the hospital to see Demi.
She shared: ''I prayed, of course, all the way to the hospital, and my faith is strong. I think that was one of things that got me through the next couple of days when she was in critical condition. We just didn't know for two days if she was going to make it or not.''
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