Will my mums cat be effected by this at all?

She is missing the other kittie when we gave my cats kitties away she became very protective of the one we kept and your moms cat will be sad and might become less active and can die of broken heart as well as dogs what you have to do is show her a lot of love scratch her head gently (spoil her a bit), maybe get her a tow and if you don't mind getting her another friend she can be with and play with the rest of her life which is still a long way to go.