Yes, it's an awful way to finish a full and valuable life. Mum's got vascular dementia, so I can both empathise and sympathise!
On a practical note, if she still has some capacity, make sure that you have Lasting Power of Attorney sorted out both for finance and health and welfare. Without these, life becomes very difficult. My advice, too, is to make sure that you get 'joint and several' for most of these - it can be really difficult if you have to have a joint decision every time you want to buy her a new pair of knickers!
It's a good idea, too, to make sure that she's made her will and that there's a funeral plan in place. This is not because it's all 'doom and gloom', but eventually she will be unable to make these decisions for herself - much better if her affairs are in order now.
The person who is 'taking up the slack' is going to need a lot of support. Make sure that s/he has at least a day 'off' every week, for the sake of sanity, and get this in place now, so that your mother is used to the routine. Also, work out what's going to happen when that person needs a holiday. Talk to your relatives about what is going on - the disease often provokes friction and resentment in families, even when they have formerly been very close!
There should be/have been an assessment done on your mum's needs by a social worker. If this hasn't happened, make sure it does. This is where a care plan can be set up. If your Mum has savings, then she will likely be a self-funder. This means it's up to you to sort out the care. We found that going through an agency was much easier than going through the list that the local council provided for us. If we'd hired people ourselves, we would have been required to sort out their pay, their holiday entitlement and their pensions. An agency will do this for you - which is why it costs a bit more. Also, an agency should have carers who can cover for colleagues who are sick or on holiday. Ask around to find out which is the best care agency in your area and look at the CQC reports on those that are suggested. We found a newly set-up agency and that was great because they had no preconceptions about how the care would be delivered. We had one carer who used to take Mum to the hairdressers and 'Singing for the Brain', which she really enjoyed up to the time when she needed to go into a care home. If she doesn't have much in the way of savings, she will be funded by the council and they will decide what help she will get and from whom.
Care homes .... yes, depending on your mum's circumstances, she may well need to move into one of these at some point - possibly quite far into the future, possibly not so far. We had the diagnosis in October and Mum needed to move nine months later, for her own safety and the family's sanity! (She used to go out and forget why, get lost and eventually find her way back a three or four hours later; there would be days when she forgot to go to bed; she forgot how to wash and clean her teeth; she forgot how to use the phone!) Have a look at what's available in your area and, again, get personal recommendations as well as looking at the CQC.
There are phones you can get which you can put pictures on and put the relevant number in the memory, so that if she needs to talk to somebody, it's easy for her to get in contact. Have a look at the OwnFone, for example:
https://www.myownfone.com/
Does she have a walk-in shower? This is likely to be come a necessity if she's to stay at home and a stairlift, if appropriate, is something else to think about.
Finally, AgeUK can be incredibly helpful - give them a call or look at their website, and you will get lots more advice at the Alzheimers.org 'Talking Point' forum
https://forum.alzheimers.org.uk/?_ga=1.243731074.602638050.1488061276
Good luck - and remember, there will still be good times. Make the most of them!