JennyC wrote:

Friends of ours have a young pear tree. It produced one pear last year and
two this summer. The problem is that while they look 100% perfect on the
outside they are all brown and rotten inside.

There is no sign of an insect or parasite. I've googled, but not found an answer.
Anyone have any ideas please?

It is probably a nutrient problem exacerbated by prolonged, hot dry
weather. The condition is similar to bitter pit in apples and caused
by poor calcium mobilisation within the plant. It most commonly occurs
during long periods of hot dry weather where near drought conditions
persist. Adding more calcium to the soil will not work since it is the
inability of the plant to move calcium around that causes the condition
and not a deficiency within the soil. Ensuring there is adequate soil
moisture during the formative stages of fruiting is the best
preventative measure. Heavy mulching in early spring and working in
plenty of organic material when planting help maintain soil moisture
levels if addition irrigation is impractical.