Sounds like you've already made your choice, but thought I'd add my 2 cents.
It sounds like you are looking for a very inexpensive option as well, so while your choices are many in the sub-$600 range, they all are about the same in quality - pretty poor.
There are a couple considerations I might mention, before you make your choice: Is your mother a heavy person? Do you plan on making long rides (several hours), such as horse camping? If your answer to those is "Yes", I would recommend a "real saddle", rather than an immitation, such as synthetic western or a cheap Australian (all of which are made in Asia where materials and labor are cheap). Neither saddle will hold up well under hard use, such as the situations above, and will leave both the rider and horse uncomfortable.
The best option, in my opinion, would be a good custom built western saddle built lightweight for trail riding. From a good saddler (example of a high-end trail saddle: Past Saddles Made
These are made with wood/bullhide trees, and have the leather components cut down to the minimum for weight. They are supremely comfortable for both rider and horse. The lightest you are likely to see one is in the 30# range. Not cheap, they can run in the $3-4,000 range, however a good Aussie saddle will run about that as well. Either will last a lifetime and go to her grandkids and theirs.
Another option I would consider is a McClellan saddle. You can't get any lighter. A child can handle one with one hand. There are makers who have modern versions of these available for reasonable prices. They are very comfortable for both rider and horse. We have one member on here (see "The Great Horse Trip: https://www.facebook.com/TheGreatHorseTrip
) ,who is riding across South America on one right now. The drawback is that they don't have anything to keep you in the saddle if the unexpected happens (which you can depend on happening).
Personally, I have an aversion to unnatural materials on a horse. Rather, I would keep a lookout for a good used saddle, to keep the price down, rather than buy a cheaply made one. You can get a decent older Simco, which is a good saddle, regardless of the fact it uses a synthetic tree, for under $500, and they are pretty light, depending on the model. They are often used in riding schools, because of the light weight and low cost.
Here's a saddle I like, for what you are looking for, from the site linked above. The second is another option you might consider, which is currently offered on ebay (no relation to me).
Just make sure you get a saddle that fits the horse and your mother (bar width, seat length, etc), or neither will be comfortable, regardless of the cost.