In my experience these are small, discrete, low-noise electrical appliances approx 300 x 200 x 150 mm. The employee will need a socket close-by and space at their workstation to position the device (it could probably go on the floor if the tube to the face-piece is long enough) I believe the only matter of concern would be electrical safety of the appliance and potential for trailing cable. Not worthy of Risk Assessment per se IMHO - that request is often the knee-jerk reaction request we get when other people have a problem they've no knowledge of! There may well be attendant health and capability issues with the individual requiring some level of adjustment, sensitivity and understanding. As always, the important thing is to discuss this with the employee.

Thanks for the replies, Will have a read of the link