There is no magic potion spray that replaces proper brushing and combing. If they are in schnauzer trims, then the legs and belly and beard need to be thoroughtly brushed with a slicker, then combed with a metal greyhound come, at least 3 times a week, removing all knots and tangles all the way down to the skin. I would suggest that if she is having too much trouble keeping them up, to just keep the entire body and legs short, and do a schnauzer face. Then all whe will have to do is keep the beard brushed and combed out. They could go as long as 10-12 weeks if cut short with a 7F blade between groomings. Stretching it farther than that is too long IMO. Using a brushout spray can make it easier to get a brush and comb thru the coats, however, it does not and will not prevent matting, or make it so you can wait longer between brushing and combing sessions. Have your mom ask the groomer when she takes them in next to show her how to properly brush and comb them. Groomers love to teach owners how to maintain coats between groomings, as it makes for less stress on everyone when coats are matt free. 
