Well first of all you will need to be careful picking out a hotel - since many in Paris are old and will not have access for a wheelchair (even if only a couple of steps at the entrance.) In terms of rooms you will need to explain your requirements - since many triple rooms would not have space for a motorized wheelchair as well as the two beds, other furniture etc. And naturally you will need to find one with a wheelcahir accessible bath. So I think you need to work on hotels ASAP to find something that you can use in your budget. (Paris has many older hotels which will not be accessible.)

Also transport on a motorized wheelchair will I think be limited to buses.

I would start now searching web sites for handicapped travelers to see what info you can find.

This David Lebovitz blog entry is 4 years old but it will get you started:

http://www.davidlebovitz.com/2008/02/accessible-trav/

Also www.parisinfo.com has a section on accessibility. Paris, I think, is a challenge for visitors in a wheelchair, but with some planning, is doable.

Several hotels listed in the 1st if you scroll down.
http://www.globalaccessnews.com/parishotelsurvey07.htm

If you have someone who speaks French, you might want to contact PAM77:

http://www.pam77.info/root

Most of the city buses have ramps for those in wheel chairs. 90% of the RER stations have wheel chair access as well. There is a good pamphlet with all of the applicable information but unfortunately, it is only printed in French;

http://www.ratp.fr/en/ratp/r_34591/guide-accessibilite-des-reseaux/

i can't help with hotels, but i would agree with the idea to stay in the 1st, and i would defintely try to be near the Tuilleries gardens - long, wide flat paths, which should be ideal for a wheelchair. this would give them access to the outdoors in an easy way, without having to access any transport.

the other place to try to be near is an RER stop - the RER is supposed to be wheelchair-friendly, and would give you access north to south [RER B] and east to west [RER C].

Both the Louvre and the Musee D'Orsay have wheelchair access

http://www.epinions.com/review/trvl-Dest-Europe-France-Paris-Louvre/content_80286944900?sb=1

http://www.musee-orsay.fr/en/espace-particuliers/private-individuals/disabled-visitors/practical-information/facilities.html

hope they have a great trip!

If you google disability access in Paris, many references come up. Many cafe and restaurant toilets are either up or down winding stairs, but certainly not all!

With careful research and planning all should go well but they need to get going on the hotel ASAP.

Several years ago I was temporarily wheelchair bound but went with DS and DL to Nice, Paris and London. Not my first trip but first to those places for DL. Since I was fairly mobile other than not being able to walk, I didn't have special criteria to meet regarding hotel rooms etc. I did want to mention that we managed very well. We did the Eiffel Tower, a boat trip on the Seine, the Louvre as well as plenty of 'walking' around. At the Louvre we were waiting in the usual long queue when we were flagged to come up front. They then gave us two free admissions ( one for me in the wheelchair and one for person pushing it). This happened several times during our trip. People were most accommodating and very helpful.

Great recommendations for my future trip.
Chevre - I hope your friend will have a wonderful time in Paris. Godspeed!