Home made wine -- Great stuff I can send you the information to make if you want -- I have made it for years Hic!

Compost heap!
I work in an office where every year someone brings in bags and bags and bags of plums from their allotment. They sit in the kitchen for a week or so, slowly rotting and encouraging fruit flies until someone has the courage to throw them into the food waste recycling bin.
I see absolutely no benefit to the grow-your-own movement if you are growing foods which you either can't or won't eat - it is a waste!

Now how difficult would it be for you to suggest the person ask if anyone would like some before bringing them into the office, you could explain your reason, surely thats the better option instead of waiting until you have an infestation of the flies..Simples !!!!

Hi 123 and welcome to GoY. I don't think there is any way you will be able to ripen the plums, sorry. How about a green plum chutney?

Wish I was within reach of your office Urbanite. Don't knock the plum bringer - the tree was probably on the allotment before (s)he took it over.

The poor questioner wanted the plums and is asking how not to waste them after all. Its such a shame.