I have made up two similar examples below.
(1a) John is a lazy student. He is not trying as hard as he can.
My non-native English speaking friends think revised my second sentence to make (1b) below.
(1b) .... He is not trying as hard as he could.
My friends think "could" sounds better. I don't understand why they are using that. I think "can" is better because my entire example is in the simple present.
What is your opinion? Thank you very much for your time and help.