greenmaid, it looks like a condition called Blossom End Rot. It affects tomatoes, too. The science of the condition is understood - the plant is unable to distribute sufficient calcium to the fruit via its internal system. It doesn't mean there's insufficient calcium in the soil available to the plant's roots. In fact, that used to be the suspected cause and the diagnosis was always to add calcium to the soil. Science has since disproved that lore. The problem is that the plant isn't distributing the calcium in sufficient quantities internally.
What isn't yet wholly understood is what triggers the condition. It's thought that a plant being stressed - hence upsetting its internal equilibrium - is a major factor. Now, a plant doesn't need to be traumatised to be stressed. We're talking small margins. Outdoors, strong winds and fluctuating temperatures can cause plant stress. As can irregular watering patterns.
Indoors, irregular watering can also cause it, as can fluctuating temperatures, but overwatering and overfertilising can both be factors. Peppers - and toms, for that matter - can have too much of a good thing. A bit like one of us eating too much rich food.
When you say your partner has tended the peppers like babies, how often were the plants watered and fed?