Quince

Yes you can put it back over heat. I usually like my quince a tad firm, especially if using as a poached fruit over yogurt or granola. If it turns to mush, then I make it into jam.

I find that the quince I buy in the USA can be rather tricky - and if you're poaching them, the larger pieces or halves will take a bit longer than you might expect to cook through. If you're not using much sugar in your recipe, there isn't really much of a dead giveaway unless you poke one with a knife for tenderness. Of course, if it's a recipe with sugar, you'll see them turn a rosy pink or even brick red when they're cooked.

I just made 1 kg of quince paste from a recipe I was given in Portugal, and despite being fiddly at first the quince cooked quite nicely and turned bright crimson red. However even then there were some hard bits I had to spoon out when I was transferring it from the pot.

Where did you/he put them away, please?

And no harm in trying! (PS When Quince are cooked—and they always take longer than I think they will—one of the ways you can tell they're done is if they're slightly pink!)

i think it would be fine if you put them back over the heat to finish cooking!