My wife and I suffered from this type of thing for too long. I'd have my share of pints etc, whilst she sat and had her lime and soda, and her mate would be on the large glasses of wine. We'd order reasonable stuff off the menu while 'friends' filled their boots. Come bill time it would 'lets just split it down the middle eh?'My sister and her other half will suggest we split the bill down the middle before family meals, and then order the most expensive stuff on the menu, adult meals for their kids, those schooners of gin, fruit and poncey mixers that cost about a tenner a go, and then expect us to subsidise it. When we took my dad out for his birthday I coughed up about £350 when we'd only spent about £150 on the stuff we'd actually had. Suffice to say I'm wise to this now though it does seem mean-spirited totting up our own tab on these occasions.
I'd had enough, and made it clear we pay for ourselves, thank you very much.
Sounds petty in some respects, and in a one off I'm OK with it, but I don't like being conned on a continued basis.