I wouldnt draw their attention to your contacts payment terms as you’re going formal then. Save that for the next level should they ignore this message.
It’s great that you have a relaxed relationship too. It makes it even easier. I’d just say something like…
Hi Sam. Payment for February is now past due, could you look at settling this soon for me please. I hope everything is OK, please give me a call if you’d like to discuss anything.
At the end of the day you need to be quite blunt as they’re in the wrong and this is how you make a living. What you don’t want to do is be too heavy handed and formal at this stage. Keep everything in writing too.
They might have a standing order. You’re jumping to conclusions without letting them tell you why they’ve missed payment. It has just been the longest January ever so they might just be struggling. After you’ve found the reason you can then chat to them about options to prevent it happening again.