There are several websites that can help you find travel for single seniors. Some of those are elderexpeditions.com, jetsetter.com, and suddenlysenior.com.
There are some great senior travel clubs out there. To help find the one that interests you most check out www.sunstoneinteractive.com/.../senior.../single-senior-travel-clubs/.
Depending on the senior travel club that you would to join, contact them and see what benefits and special details are that they have for single senior travel. All companies will be different so be sure to contact them and find out their information.
You can find information on single senior cruises for your grandmother by seeking travel agents who specialize in senior vacation packages. Very few general travel agents or online booking sites offer this service.
A lot of information is available on the web regarding senior travel clubs. One place that you will find helpful is at www.suddenlysenior.com/TRAVELPAGE.html
You can find many singe senior travel groups on the internet here is one (http://seniortravel.about.com/od/tourgroups/tp/Tours-for-Singles.htm) Senior travel is the one of the ones you come across plan your trip with them!
There are airlines that offer discounts to senior citizens but you have to find them. here is a good place to start: http://www.onetravel.com/travel/senior-travel-deals.asp
There are senior travel packages. Go to www.seniortravelhub.com/senior-travel-deals/travel-packages or www.tripspot.com/seniortravelfeature.htm to find these packages.
There are a lot of resources that about senior travel tours available online. One place that you should check out is at http://seniortravel.about.com/
There are plenty of places that you can look for cheap insurance for travel for the elderly. One place that you can obtain the insurance is at http://www.mrlinsurance.co.uk/travel-insurance/senior-travel-insurance
Yes there are many traveling clubs that cater for single seniors like your mother. You might try to check online and visit http://www.grandparents.com/gp/topics/single-senior-travel-groups.html there are very many resources that can be helpful to your mother on this website.
You can find information on senior travel insurance at http://www.moneysupermarket.com/travel-insurance/. Hopefully there you'll find what you are looking for
One can find more information about senior travel on various places across the world wide web such as the Senior Living website and Senior Travel on About Dot Com.