Rhododendrons

My daughter has just pruned her very big rhododendron bush, in her garden in Guildford. A real family favourite!   Its a fabulous plant, has lived in its massive 1 metre square clay pot quite happily for probably 4 years; topped up with fresh rhodie soil twice a year, & fed regularly;  it's been tended & looked after & "thrives" & flowers profusely, (gloriously, every year) & its kept well watered all year round (so gets huge fat buds in between seasons)  - but as it's probably all of 5 ft across now, (has never been pruned) this year she felt it should be pruned.  Whilst I realise the "horse has bolted" - any comments/advice on tending this beloved shrub, would be dearly appreciated. (Am told it is looking even healthier since it was pruned this week!)  I visit for a week every month or so & they enjoy my involvement with helping them with the gardening there...hence my interest - thankyou  question:  when IS the best time of year to prune a rhododendron?