Do not underestimate the loneliness of widowhood. Your mum may seem to have "carved out a nice life for herself" to you but this is how widows cope .... what else can they do? They can't sit around weeping for the rest of their lives and being a pain to those around them. They have to find a way of cracking on.
Please do not be lulled into thinking this means she is happy or that the huge well of loneliness is not still there. Living alone after a lifetime of coupledom is very hard indeed.
Please also bear in mind (and I say this kindly) that your assertion that she has a nice life might also be what you need to believe or to see so that you do not have the burden of feeling responsible for her happiness. The fact that she is making this decision now belies the idea that she has truly been happy.
Maybe she likes his dominant ways; maybe she is tired of being responsible for her own life with no-one there to bounce ideas off, to hand things over to when she has had enough.
This is not a sudden friendship .... she has known him for years so feels confidence that she can trust him.
And there may be ways that her having this man in her life might also make things easier for you.
From her point of view she gains nothing by waiting.... her waiting time is limited ... she is in death's waiting room already and she knows it. She is grabbing a chance of happiness.
You probably feel that she could have the benefits of his companionship without marrying, but it clearly feels important to her. She has soldiered on alone for 10 long years.
This is what I think you should do .... congratulate her with a big hug then at some later point ask if there is anything that you need to talk about with her as her PofA to be sure that you get her financial and care wishes right in this new situation. Do this in an objective rational way rather than with any hint of being aggrieved.
She is a grown adult and is allowed to make her own decisions and her own mistakes.
I am not sure that these are the "words of wisdom" that you were hoping to hear but they are from the heart as I too am widowed and all the world thinks I am coping so well because that is what I choose for them to see as I do not want my family burdened with worry about me. They do not know the daily pain and sense of loss that are the underlying threads running through my life.
Be happy for your mum.