お久しぶりね!

小さい質問あるの..

I got a message from a Japanese acquaintance asking about going to dinner this week, and I noticed that she switched between the informal だ and the formal です in the message. Now, I've seen people do it before, but whenever I do I'm corrected for it. Is it normal for Japanese people to do this? Is it considered 'correct'/acceptable?

ありがとう