I would not take that advise as red, it will need at least 100mm from obstructions but it does not mean that you can box it in the issue as seco intimated it needs free air flow.
You can create an attenuator to quieten it which allows air flow but deadens noise but it cannot be enclosed, if you enclose it especially being an RHP if you get the motor windings to over heat then it is knackered from there onwards.
I think you will find that the cupboard might be more the issue, my guess is that it is plasterboard in general construction.
The RHP100 is probably the one of the quietest 3bar whole house pump you can buy.
Things that are worth doing,
a, under the pump first put 50mm foam or polystyrene insulation, then use a paving slab and rest the pump on top.
b, do not clip the pipework to hollow walls just put pipe lagging on them,
c, do not have 90 bends close to the pump on the outlet side.
d, put pillows duvets etc near the pump (nothing worse than an empty cupboard).
e, and probably the best idea ask the wife to move out!! does not always work but will save a lot of money.