Kudos to you for wanting to take this on yourself. I believe basic bicycle maintenance is something everyone should know. I obtained several years of experience as a bike mechanic before and during university, and am a former mountain biker racer who went through a lot of parts - so I've serviced many different kinds of bicycles and associated parts, from basic to advanced.
Which, unfortunately, is why I must say that you may find yourself past the basics here and more into the intermediate to advanced, depending on what kind of bike she has (i.e. how the rear cassette (coggs) are setup on the bicycle), and the actual problem of the rolling resistance. This could potentially be a time consuming, expensive repair here - even if you have the right tools.
It isn't just because of the age, and the fact that you're starting to notice rolling resistance which will likely require replacing ball bearings along with the cleaning and regreasing of the front and rear wheel hubs, but also perhaps the replacement of an entire hub body (which means re-spoking the wheel with a new hub, or maybe even buying a new (or used) wheel if that is cheaper).
Add to this that this requires special tools, such as cone wrenches to unlock and relock nuts that hold the cones to the bearings, as well as the cassette free-wheel removal tool and chain whip tool if necessary.
First and foremost, make sure tire pressure is appropriate (as written on the side of the tire) and that the brake pads have not shifted somehow and are rubbing against the rims continually. Both of these things lead to rolling resistance (though perhaps a good workout as well? :)
In any case, found an amateur video that might give you a sense of what you're getting into here.
http://www.youtube.com/watch?v=IrzlHjhckSUAs for the grease - and any of the parts/tools required - you will in fact need to head to your local bikeshop. These are not automotive parts or tools, and bicycle greases and oils are generally made to shed dirt/moisture as they're exposed to the elements, unlike automotive grease/oil, which is meant to stick to everything (or have everything stick to it - not something you want on a bicycle).
Something to look out for, if and when you decide to do it yourself and get it apart: once you have cleaned the hub of all debris/old grease/dirt, verify that there are no cracks inside the hub where the bearings sit. If there are, even if minor, you will have to replace the hub. If you do not, then all your work will be for naught, as the bearings will begin to breakdown again soon from the grinding through the hub cracks, no matter how well greased.
P.S.: Skewers do not typically interfere with rolling resistance, as they're not turning while the bike is in motion (the axle itself does not turn while the bike is in motion). If they are interfering, it is because the wheel has fallen off due to someone not tightening the skewers enough before riding - kind of hard to roll well without wheels. ;)