Money Moral Dilemma: How can I stop my friend 'forgetting' to buy rounds?

I drink in several different groups with differing payment methods. Where we all drink about the same speed and similar value products, we buy in rounds. Easily remembered as we go by alphabetical first initial, then if one forgets, someone in the group says ‘who got the last round’ and we take it from there...the next in the round usually gets the hint before someone else points it out to them. If the rounds cycle back to the start & someone buys a 2nd round, the next time we go out together, we use the same principle. You quickly become familiar with who you follow in the group, and who comes next after you!

Other groups have different speed drinkers and buy their own. I often drink water between rounds (Drs advice) so ‘going Dutch’ suits me here.

In another group (boating trips) we have lightweights, ‘par’ and heavy drinkers. Here, we all put £20 in the kitty then buy rounds, each adding further £20 as the kitty runs down. If we all have a beer, nothing is written down. If someone has a half or skips a round, they get a credit. If someone has another beer between rounds, they get marked as an ‘extra’. At the end of the trip we divide any remaining cash balance after adjusting for extras (putting more £ in) and credits (more cash out). In practice, money does not change hands but offsets costs elsewhere for boat hire, food purchases etc. A simple spreadsheet keeps track. After a year of boat trips, beer exploring abroad, etc, typically < £200 actually moves around the group.