My friend just finished college and I feel like I should let her know that her degree is useless
That's not very encouraging. Why would you want to tell them that.
She should know the truth of how useless it is.
It is rude to make your friend feel bad about their accomplishment. You should be supportive or just ignore their degree.
It's her fault for wasting 4 years of her life. She should feel stupid.
A lot of jobs just want to see that you managed to get some kind of degree, so I'm sure it's not totally useless. Your friend will want some encouragement as they start looking for a job after school. Saying something like this will probably push them away.