I'd have her really think about this commitment.. At 20 I can't imagine she will be living at home for TOO much longer, and it is hard to find a place with a cat. Cats can live say 15 years. Does she envision having this little thing for all that time? What about traveling etc?
I got a cat at 25, and when my mom pointed all this out I realized she was right and I found it a loving home as I cannot commit to giving up my footloose and fancy free lifestyle.
"Life is full of beauty. Notice it. Notice the bumble bee, the small child, and the smiling faces. Smell the rain, and feel the wind. Live your life to the fullest potential, and fight for your dreams.