Of course a retired person is available for work if they want to be.
Lots of people continue working after they have retired from one job. You are only retired permanently if you chose to be. Being retired is not the same as being eligible for your state pension, in which case you cannot claim JSA although you can, if you chose, continue to work.
If she leaves her job voluntarily she may not be eligible for JSA. Enquire at the DHSS
She is not technically retired until she reaches her 60s. I believe in her case 67. if she leaves her job she won't get JSA for 6 months as she has not retired but voluntarily left her job.
She needs to keep working, maybe she can go part time.
UK
Greywolf is right - if she voluntarily quits work, she'll only have her private pension.
And I doubt she'll get the state pension at 65. I won't get mine until I'm nearly 67!
What other benefits she'd get depends on what savings she has.
You can't get JSA unless you are "available for work". A retired person is not "available for work". But get her to visit Citizens Advice Bureau. It's free and they are on your side. There may be other benefits she would be eligible for.
she shouldn't be retiring if she has a child to support and 55 is way too young to retire