I should imagine, as Bright said, that they would be paying for it jointly, since they're both adults.
I don't think the OP is at all responsible in that case, I think if the husband loves his camera that much, he should have been more careful. You suggesting 'going halves' suggests you don't think the OP was entirely responsible either, however most agree that the boy was entirely responsible in this case.
Also, in that case, the OP described £60 as 'not much', whereas in this case the OP seems to think £70 is a lot of money. Perhaps a financial difference?
I think in the camera case, the post was more about the immature reaction of her husband. They were both adults.
Who was financially responsible didn't really seem to be an issue.
In this case, it's the financial responsibility that is the important thing (as well as the boy's reaction).
The camera was bought by the husband, or jointly as a couple, and then broken through his carelessness.
The PE kit was bought by the parents, for the son, who then lost it through his carelessness.
With the camera, they can choose to buy another one as a couple, or not, if they decide they don't need it.
With the kit, the boy needs it, so his parents have to provide him with one, therefore they have to go to the expense of buying him another one. The OP is debating asking the boy to contribute to this.