Cherry blossom tree advice please

Ooo - nasty, FC. It might be worth Googling 'Cossus cossus'. That's a huge moth that lays it's eggs at the base of trees, especially plums and cherries. The larvae crawl up the trunk and bore into it, then proceed to eat it from the inside for 4-5 years, after which they pupate and start the cycle all over again. I don't know if they're present in the UK but it's worth checking any suspicious hole of 1/2" diameter with traces of sawdust underneath. It looks to be way too late for this tree. Best to take it down before it does cause some damage.

 

Looks to me like bad pruning long ago has let rot in.
You can see how the bark is curling in, in an attempt to heal the gaping wound.
I dont think you can do much about it, as you say FC.

I get the feeling it might have a few years left in it, but it will always have the potential for crashing down at some point.

 

IMO the tree has (regrettably) had its hey day. The heartwood aappears to be pretty much non-existent in the pics. The tree may still survive a few more years but is on borrowed time and could be a liability if it falls, as it appears the weakest point is near the ground, so pretty much the whole tree will come crashing down.

Only last weekend it was windy and a large, heavily-blossomed Damson tree nearby had fallen over right across the pavement, crushing a tall privet hedge. The tree looked in great health apart from what appeared to be a couple of Artists fungus near the base of the tree. Looking inside the broken stump, it was rotten but still complete internally. Yours looks in a far worse state

 

Eeeek! Bad news. Like I said, a heads-up for anyone reading the thread that has fruit or ornamental trees. The only cure is to stuff a stiff wire up into the channel (s) caused by the larvae and squish the blighters. There are pheromone traps available but they don't seem to be very effective.
Your friend's cherry tree wouldn't have a preservation order on it, so no problem taking it down.