It's definitely hard when you witness someone existing in a low who doesn't acknowledge it as being a problem. I suppose one way of looking at it involves considering various levels of depression. Imagine in this scenario there are 5 levels (I'm making up that number just for the sake of basics). So, you might start at level one where you begin to lose interest in things that don't give you much satisfaction anymore. Could be due to chemistry issues like hormonal for example. Then you shift down into level 2: You begin sitting around a bit more watching TV, due to the feelings of dissatisfaction on top of gradually decreasing energy levels.There is no interest in going out anywhere and life appears more comfortable simply staying at home. Then, suddenly, you find there are no social contacts, the only thing you're really interested in is the news and you've put on about 10kgs because chocolate is the only thing that gives you that hit of dopamine you crave. Then BAMM, you've slipped into level 4 where life holds no happiness and you feel like an unloved worthless failure. Level 5 is like hell on Earth. Been there and wouldn't wish it upon anyone.
So, even though you can see your mum slipping into a deeper stage of depression, it may not be so obvious to her.
Consider introducing new things. The new things can be incredibly simplistic. The aim is to guide her into experiencing highs, without necessarily revealing to her your plan. For example, the first thing you could do is give her a pack of her favourite biscuits that she hasn't had for a while. Maybe the next thing is to give her a couple of colourful plants to plant in the garden and take care of. Perhaps what comes after that could involve you saying to her something like 'You know what mum, the other day I tried a few stretching exercises in the chair and it led to the most amazing humming or buzzing feeling in my feet. Have you ever noticed this feeling? See if you can feel it. C'mon, give your feet a bit of a stretch and your calves a bit of a massage. I'm not asking for much here'. Curiosity might get the better of her. Basically, what you hopefully end up doing is bringing her back to life through sensory experiences.
Eventually, you could challenge her to come up with 1 new thing every day, herself. It could become a habit. You notice, you're not telling her to reconnect with friends or do anything quite challenging. You may raise her to the point where she considers this herself.