my ds is 10 (year 5) and I am passionate about letting him be independent in a safe context and teaching him bit by bit to be streetwise. He is very sensible and knows the rules we use to keep the boundaries. He goes to library, to his friends and to and from school (only round the corner)
Would I let him do this with a friend? Not sure. Our equivalent would be to go into town on the bus, and home again.
I actually think he and his friend would be fine, and love the sense of independence. In 18 months he will probably be getting on a train to school every morning, and at some point will need to bike or bus to the station too.
But I don't think I would let him just yet. he is year 5, but I would if he was year 6, as a part of building him up for secondary.
I like the idea of dropping them off, or letting them go on the train and then picking them up. Make sure they have a phone and a default meeting place if they get lost.
I would be happier about it when there are two of them.
I wouldn't be worried about them shopping, as I would assume they didn't have much money, so if they want to buy a pair of leggings or a T-shirt it isn't the end of the world.