New boyfriend was a dick. Sorry, but yeah, that's a dick, childish move. I'm sure the ex husband was probably a dick too. But at least he was paying for his kids. If it had been me, I'd have gone to the bar my damn self and cashed the ex's cheque and had a drink, and then left to look after my kids, but I'm petty like that.
I remember those times, cashing cheques so easily, buying cigarettes as a kid with only a note (forged) ...ah, the good old days. :)