I'm going to tell my girlfriend she can't assemble Ikea furniture on her own and she needs help
Why do you want to tell your girlfriend that she can't assemble the furniture?
She has been assembling it for awhile and keeps messing up.
You should not tell your girlfriend what she can and cannot do. Instead, you should ask her if you can help her to assemble the furniture. By helping her you will be able to properly put the furniture together.
I'm only looking out for her and believe me, she would be glad that I told her that she needs help.
Well then help her out. You could offer kindly to help then take over the project and do the more difficult aspects while she holds things in place or prepares the screws.