This Information is provided by NHS Choices.

How can friends and family help?
If a member of your family or someone you know is a compulsive hoarder, try to persuade them to come with you to see a GP. This will not be easy, as someone who compulsively hoards may not think they need help, so may not want to seek treatment.
Ask your GP to refer you to your local community mental health team, which may have a health professional who specialises in OCD and is familiar with hoarding.
If you have difficulties accessing therapy, the charity OCD-UK may be able to help.
Reassure your loved one or friend that no-one is going to go into their home and throw everything out. You're just going to have a chat with the doctor about their hoarding, to see what can be done and what support is available to empower them to begin the process of decluttering.
It's generally not a good idea to call in the council or environmental health to clear the rubbish away, as it won't solve the problem and the clutter often quickly builds up again. However, if the problem is not addressed, local authorities may take legal action to ensure that the property is cleared.
Do not be tempted to get extra storage – this is only a quick fix, is costly, and does not address the problem. The problem is too much stuff, not too little space to keep it.