Hi there dsheffield,
I'm actually in exactly the same boat as you - only child, recent Alzheimer's diagnosis, mum living on her own after 20+ years divorced - only difference is I live outside the UK now. But I had the same dilemma when I was back last week. I brought it up with her and she seemed pretty receptive - I chose the local Crossroads, and she came down with me to their offices. They were lovely and really sold the idea to her. She also realised I think that she needed help.
She started with them this week, including their day club which she absolutely loves. I think I might have been quite lucky at her receptiveness, I think it's all about how you 'sell' it to them. Don't make it out to be a care worker, just someone to have a natter with and go for a walk with. She's quite lonely so the idea of someone coming round most days for a cup of tea I think seemed quite appealing.
It also means I have 'eyes and ears' keeping an eye on her from a great distance - it's as much of a comfort for me as it is her.
Hope that helps,
cheers
Alan