The YWCA hotel would fit the bill. It's on the Airporter route and it's about 2 blocks from the bus stop for the Horseshoe Bay Ferry bus.

www.ywcahotel.com

Also tell your mom to take the 257 bus from the ferry terminal unless your mom wants to spend time going through West Vancouver.

I'm not as familiar with hotel prices in the city, but there's a list of hotels on the Vancouver Airporter's website of hotel's they stop at.

http://www.yvrairporter.com/MapHotelList.html

Lots of good hotels right on the route. The airporter takes a convoluted route through downtown and the west end. The most convenient hotels are on the beginning part of the route as the bus lets people off there first. Some of these hotels are: The Quality Inn at False Creek (which has a very good deal going right now if you book through Travelzoo's portal--they give a promo code on their website [travelzoo.ca] for a goodly discounted price of $89 per night), The Best Western Downtown, the Executive Vintage Park, the Marriott Residence Inn, and the Sheraton Wall Centre. There are all good hotels, and I have stayed in most of them over the years. I would try the Quality Inn at that low price--it is decent and well located.