I have read that there is a lift to take people up to the surface at the end of the visit, but can one take a lift down? I am going with a disabled friend who can't manage stairs. Information on this would be much appreciated

I hope it's not too late. I don't think it should be a problem, but you should contact them and ask: http://www.kopalnia.pl/site.php?action=site&id_site=169&id_language=2&site_location=3&deparment_change=true&
From what I've seen there's only limited tour a person in a wheelchair can do, most walkways are uneven and not suitable for wheelchairs. The regular tour takes three hours, that's three hours of walking. I think you better email them. Good luck.