How to adjust SR SunTour M3020 AND product manual
The entry level SR Suntours don't have a lot to offer as far as adjustability. I think most of the lower level ones such as the one you have are preload adjust only. Some may not even have that. Basically if it has a knob on the top of one of the legs turn it all the way to the right. Squish the fork a couple times, then turn it all the way to the left and do the same. That is your range of adjustment, this will change the "sag", which is the amount the riders weight alone makes the suspension dip, it should be set to about 15 - 30% depending on conditions and preference, find what suits her best. 

Unfortunately mushy perfectly describes SR Suntour's entry level product. I'm pretty sure the stanchions are just filled with mashed potatoes and sent out. They work for small bump compliance but that's about it. Although there is a big step from the M series to the XCR &amp; XCT series, they are by no means great forks but they seem to be a lot more fine tuned than the M series. You can find one for about 100 bucks in the states.