Certainly some sort of Aesculus (Horse Chestnut). Red Horsechestnuts (various hybrids) have smooth conkers as do some of the Buckeyes, afaik. A pavia, Red Buckeye has similar fruits according to the interweb. Presumably there are still plenty of leaves on the tree so how about your friend looking at Aesculus pictures on Google to find a match?

BTW The common Horse Chestnut has two nuts in each shell so this one is not unusual in that).

Yellow Buckeye (Aesculus flava) also has smooth husks.

I think A. flava is pretty rare to find planted in the UK, however.

I'd guess a. pavia is a good candidate. I grow them and have them planted in numerous places on my property and the husk and nut are similar to your picture. Nice tree, quite desirable and smaller and thriftier than other buckeyes.

thank you all!