Hmmm. Seems to me like the Halfords employee might have been in the business of dispensing bovine guano. I have a Garmin of similar age and there is absolutely nothing in any of the user manuals (I've just double checked on-line) about charging the battery in the manner described. Indeed, the
maximum charge time required is given as 4 hours.
Neither the battery nor any other part of the unit will be guaranteed beyond the initial warranty period but if any manufacturer were to wish to rely on a particular charging regime to maintain a warranty they would have to spell it out in the user instructions.
On the more general point of changing the battery. It is described as Non-user-replaceable (as opposed to not replaceable at all), the reason being that damage to the unit might occur if an amateur were to attempt the job and/or inappropriate tools were used. It is, though, possible for most batteries to be replaced - though whether that is economically desirable might be questionable
Graham