How much does your mother exercise?? Is she piling on weight due to antidepressants? Is she employed outside the household in something too demanding or her everyday life is the opposite and not demanding at all?
As Cannuck said, it's diffucult to advise fully online.
That your father is in good health and has a good job is a plus for material comfort and that you are well settled is also good, but to assume that there's therefore no "reason" for depression is, I'm afraid, too easy.
With depression, the body stops/ reduces it's production of own "happy chemicals" these include endorphines and serotonin, for example. The person can end up in a downward spiral of lack of motivation-lack of happy experiences-does less-produces less "happy chemicals"-does less-etc etc. Being depressed and not motivated doesn't make it fun for people to be with you. No fun, no laughs, less possibility of boosting own production. The spiral continues. On a practical side if it's linked to menopause: she should start some form of regular exercise even if it's tough to start. If she has a good friend to do it with, to laugh with, all the better. Enough fresh air is also important. Can she walk a dog? Get to a park? work in the garden? Do your parents go out together? To a movie? to friends?
It's often difficult to talk about problems with your own family, if your mum has girlfriends the same age who she can chat to and be active with encourage her to do so. To improve your understanding of what women around 50 plus can experience try to look into the references you've been given or what you find in a bookshop or online, you don't have to let your mum know or deliver a lecture on it !
regards JM