Strange Fruit in Neighbor's Yard

Our neighbor has this beautiful massive fruit growing in his front garden. A landscaper by trade, he also happens to be our landlord. Whenever I see him out trimming behind our building or in the neighborhood I pepper him with questions to practice Japanese, but also to learn whatever possible about the plants and trees around us.

I'd noticed the fruit pictured at left growing on his garden wall trailing just beneath a very nice rose, and asked about it. The stem is about as thick as my thumb with what look like rather viscious spines. The leaves look like what I consider the usual citrus: slightly oval, thick and a bit waxy, and dark green. It turns out, of course, that he doesn't know.

I suspect, based on my observations and his description of it, that it is a pumelo. The skin is very thick and it apparently has loads of seeds. The taste of the fruit, though, is where his description varies from the standard for pumelo. He says it's intensely sour, whereas everything I've come across so far says that the pumelo is sweeter than grapefruit. He's not offered me one yet, and while the idea has crossed my mind, I don't quite feel right simply snipping one off the vine in the dead of night. Thoughts?