All assuming England - the rules differ between the four counties in the UK.
As stated, IF the person doing the work is competent to do so (and has the appropriate test equipment), then it is legal for them to do it. If the work doesn't include notifiable work then there's no notification to do. The ONLY notifiable works in England are replacement of a CU, addition of a circuit, work within the zones of a bathroom.
However, when you say the landlord installed his own pre-pay meter, is this in addition to (and downstream of) the supplier's meter ? It would be illegal for him to interfere with the existing meter.
There are a load of regulations that come into play when a private meter is installed, so unless he's signed up with one of the outfits that manage this sort of thing, it's highly likely that he's breaking some regulation or other relating to the private metering.