A very nice gun & I would say a quite uncommon example. I am assuming that the gun & carriage are matched & both are of Armstrong's manufacture, are there any markings on the barrel to confirm this?
It has a low serial number & I am fairly certain that this dates to the mid 1860's, certainly the style of barrel & carriage conform to this period. This was not long after Rifled Breech Loading gun designs had been superseded by Rifled Muzzle Loading guns, many would argue this was a retrograde step. Armstrong was at the forefront when it came to ordnance in England & examples of his guns are therefore plentiful, but most surviving examples are of a large to very large nature & finding an example of such small calibre is uncommon, I do not recall having even seen one before & certainly not one complete with carriage.
It does not conform to a British military pattern, the smallest RML was the 7 pr Mountain gun in bronze, then steel (this was in fact the first use of steel by the Ordnance dept as opposed to cast or wrought iron etc) & the external profiles are quite different from your friend's gun. So this is almost certainly a commercial example from his factory - assuming of course that the gun is an Armstrong, as it is quite likely to be.
I would suggest that the owner contacts the Royal Armouries at Fort Nelson to see if they can offer any solid information & also to bring it to their attention. It is a gem & a piece to be very pleased with indeed.
Adrian