I think it sounds kind of ridiculous for us that she forgot the Friends lines.. but if you think about it, it actually makes sense.
First of all - yes, she is an actress and has to learn those lines by heart. But I think once they’re done shooting, you forget them because you don’t need them anymore and have to make room for lines for the next episode.
And also - she doesn’t watch Friends like daily. I think she said she catches it from time to time, but that was 10-20 years ago and if you haven’t seen the episodes as often as we do, that explains why she doesn’t remember most of the lines.
Just something I assume. She also said that she has a bad memory in general. I do, too. I can’t remember most of the things that happen in a movie I’ve only seen once a few years ago until I rewatch it. ;)