How do I ask my friend to stop 'forgetting' their wallet?

Whenever we go out for coffee, a pint, or just to grab a quick lunch, they always seem to 'forget' their wallet or their card gets 'declined'. At first I didn't mind spotting them, it's just a few quid here and there. But I was looking at my bank statement the other day and realised it's really starting to add up.

The thing is, I know for a fact they aren't broke. They have a decent job and dont seem to have money troubles otherwise. It just feels like I've become the default person to pay for stuff. It's got to the point where I get a bit of anxiety before we meet up, wondering if I'm going to have to pay for everything again.

I really value our friendship and I don't want to make it super awkward or cause a big fight over a bit of cash. How do I bring this up without sounding cheap or accusatory? Has anyone else dealt with a friend like this? What did you do?