I don't think she did it on purpose, in the sense of setting out to sabotage him, but it was definitely an incredibly selfish thing to do, and what did she think was going to happen if she leaves ice cream out on a boiling hot day. Iain said in an interview that they don't have designated freezers anyway, and that 2 of the 5 freezers had broken down which is why they were short on space and he went over to that side of the room.
As much as I'd have liked him to stay, I can understand why Mary and Paul sent him home. He could have easily just covered his cake in the meringue and then put some of the melted ice cream in a dish next to it. It would have been more of a sauce than an ice cream, but they'd have still been able to judge the taste of it. The moment he threw it in the bin, he sent himself home. How are they meant to judge when they have nothing to judge on?
Anyway, in the long run it'll probably be the best thing that's happened to him. He didn't have a chance of winning, and this way he's got a massive amount of attention, which will probably lead to getting cooking slots on shows like This Morning or Sunday Brunch etc. He was also being interviewed on Newsnight tonight haha. Who would have thought it would cause such a scandal?