Almost all name brand sergers will be fine. If she's an accomplished seamstress you going to want to get a good quality one.
4 threads best. 3 okay but really need that 4th thread option.
Lower looper, upper looper, left needle, right needle. 4 threads.
Please get her one that has a self threading option. My Juki doesn't have that option and my father in laws baby lock does. I am so envious...lol. He simply ties the new color of thread to the end of the old color then pushes the pedal.
As the knots get to the needle hole he has to stop and cut it so he can use the needle threader then move on. It's the best thing EVER!!!
I love that option. I would not want one that only had 3 threads although I know that some decorative stitches only use 3.
I think you might want to consider a book or two that has cool serger lessons/how to sew cool stuff with your serger.
And if you can afford it get a carrying case for it. If not don't worry. She'll get one sooner or later so she can enroll and take classes at some local quilter store.