He could try treating the door rubbers with gummi pflege, it conditions the rubber as well as making it more weather proof
https://www.amazon.co.uk/s/?ie=UTF8&keywords=gummi+pflege&tag=googhydr-21&index=aps&hvadid=155840528552&hvpos=1t1&hvnetw=g&hvrand=1274872852475862958&hvpone=&hvptwo=&hvqmt=e&hvdev=t&hvdvcmdl=&hvlocint=&hvlocphy=1006748&hvtargid=kwd-2676613041&ref=pd_sl_2556zng6r1_e
Rob
Ah, just noticed it's the lock itself that's frozen, try some graphite lubricant
https://www.amazon.co.uk/Locksmiths-Lock-Lubricant-Graphite-Lubrication/dp/B008OH957A
Rob
I used to blow into my locks on my cars without remote central locking - which was almost all through the Eighties - not putting your lips on the metal or they will stay there. Lean down tot eh lock. Say 'O' and give a number of long blows. Heat the key up in hot water or something and work it carefully into the lock. Worked fine for many years, back when we used to have proper winters.