Here's a fun idea. Not exactly the answer the OP asked for; but serviceable for the character described.
Premise: Time travel IS impossible, he cannot go back, and he cannot do anything in the past that affects the future. Period.
That said, what he learns while researching time travel is a way to recover lost information from the past, in the present. We do this all the time, in the form of archeology and the study of fossils; we learn of things that actually happened, perhaps five or ten thousand years ago.
This is analogous but a million times more precise: Our hero can actually read, down to the atom, the structure of any object in the past. He can't read the quantum states of all that, but which atom is where: He can read that. He can't change it, but he can make a copy (think of a Star Trek transporter).
That applies to the brain and body of his great love. So, using a great amount of energy, he picks a moment in time when she was last healthy, and makes a copy of her at that moment materialize in his lab. A little electrical shock to reboot her quantum states, and bingo: There she is, in the flesh, with all of her memories and personality, waking up in what seems to her a hospital. She has questions, like "WTF happened to me", etc.
In the past the original continues to her death, as must be the case. But he has his love back, unchanged from the last moment he saw her.
That said, HE is not unchanged, he is (say) 30 years older, an obsessive compulsive reclusive billionaire, emotionally devastated and withdrawn after suffering through 30 years of her loss. (Heck, let's say he caused her death through some careless act, and rightly blames himself.)
He is nothing like the man she loved. Further, let's talk about everything else she loved: Both her parents are dead; her sister died of cancer. All of her friends have aged 30 years, from her point of view she time travels thirty years into the future with no way back. Everything she knew, her routines and friends and acquaintances, her work, her politics, her understanding of the world, even the man she loved, is all irretrievably amputated.
In a way, her life was still lost: She had a network and it is gone, dead and broken and moved on from her for 30 years; the people alive in it can hardly remember her. Say they were in their mid twenties and met in college: She was in love with her college sweetheart (our scientist) a young and newly minted PhD that loved his life, was excited by his quantum research, full of ideas and vitality!
She was looking forward to their wedding and their life together, the children they would have and raise, the careers they would work through together. Not some geezer older than her father at the far end of his career, somehow mysteriously rich, a recluse in some futuristic compound professing his love for her.
She is a smart girl; she understands everything that happened. but what does she do now? How does she acclimate? Eventually, after the stages of grief, she will break down emotionally and cries for all she has lost, accepting the impossibility of changing it.
How does she move on? Does she fall in love with a different young scientist?
Or here is a fun twist: She convinces our hero she does not return his love, she doesn't know him. She loved the man of 30 years ago; but clearly (to her) that man died when she died.
Our scientist realizes she is right, and what he must now do: As a selfless act, he runs his machine one more time; to retrieve somebody else from the past: Himself, from 30 years ago. And when his younger self wakes up, he tells him: "We need to talk."