I'm not a woman either.
I think a good quailty 'mixte' or 'loop' frame is your starting point: a frame that easily allows mounting the right leg in front of the saddle. But you already know this.
Charge, Soma, and a few others make modern mixte/loop style frames.
Be aware that very skinny tyres aren't necessarily 'faster', though they can affect weight saving considerably. I'd expect 25 c - 32 c tyres, on a lightish bike with really good components is your best aim here, assuming that your mum wants to ride longer distances in comfort.
Take a look at bars that have lots of hand positions, such as moustache, french touring, butterfly, compact drops, wide flared drops ...
Check out sram apex groupsets, compact doubles with wide range 10speed. Awesome stuff for the price.
Another good option could be a brompton.