Lots of this going on, sadly. If your boiler was fitted correctly originally regarding ventilation (if needed, probably not), flue, etc then it can certainly be worked on. The only proviso is that it operates safely, and is certified to do so.
An external wall mounting is not compulsory (just easier usually.) All sorts of fluing arrangements are available.
I won't try to go into it all since it's quite involved.
In short, your guy was either lazy; ignorant; or looking for a nice plump new installation job.
Have someone else look at it for you. Try and get a recommendation for someone who is trustworthy.