Grammatically, they're fine. Logically, they're not. When are you ever going to need to say this to someone? Why would you first meet someone while they're getting dressed? Would you have expected him to be naked at the party? If you saw him getting dressed and putting those other things on before the party, why would you be surprised to find him wearing them at the party (the "but" indicates a level of surprise)?

Student or Learner