Looks like one I saw in Dorset, in which case, look here http://www.nhm.ac.uk/natureplus/thread/2440?tstart=0
If it's the same, that'd make it a cherry-plum. I didn't see thorns, but there's certainly a lot in common.
I reckon Rhussel is right and it's a cherry-plum (Prunus cerasifera). The spininess is quite variable in this species - many of the plants of it you see planted are spine-free.
Thank you very much Fred, my firend Helen was concerned about the fruits because of grand children and unfortunately she has the spiney variety!
Pauline and Ann
Sorry to pick this one up late. You already have the identity of your mystery plant but might like a little more information.
Prunus cerasifera is the cherry-plum, mirabalan or mirabelle plum and can fruits of several different colours, including yellow, red and purple. These fruits resemble plums in msot respects but tend not to have a marked, greyish bloom. Just like plums, they are edible - not great as fresh fruit but good in jams, liquers etc. Your friend Helen is indeed unfortunate to have one of the spiny trees as most individuals have few if any spines.
2009 and 2010 have both been very good for cherry-plums, with masses of fruit produced which is perhaps why people seem to have noticed these trees more.