Pauline
Their UC should remain in payment and while they are in hospital they automatically have Limited Capability for Work, it might be worth their having someone remind the DWP of this. The UC50 is to assess whether they also have Limited Capability for Work.
Is he totally unable to complete the form? If he is then contact the DWP and ask for the form to be withdrawn until he is able to fill it in.
If the form still needs to be returned by the specified date then you/he/his relatives need to find a way to make this happen or he risks losing their LCW as soon as they leave hospital. However, this does not mean that the form needs to be completed in detail, speak to his doctors they may be able to provide a sentence of two for the applicable Descriptors.
If you have problems with the DWP then involve his MP
Contacting your MP Gordon
Gordon.