I'll be honest about this issue, there isnt really much you can do, if the husband is doing the adultery, then the only real thing to do is to press charges. Plus the husband's demeanor to your mom is unacceptible.
Violence is the absolute last resort if nothing else settles arguments, and even in such cases, restraint should still be practiced since men can really hurt people around them if they go berserk. My advice now since there is a distance barrier between you two, is to provide moral support, call her as much as you can, no matter how brief, hearing the voice of someone who really cares helps the morale of the person to endure whatever hardships he/she has to face
As for you, jobs are always around. If you use your resources wisely, you can turn this into your advantage, being a working student can either be hard or fun depending on which perspective you choose to look.
Lastly, dont be miserable about the husband's shortcoming, you have every right to be angry but remember that anger is one letter short of danger. Triggering hate not only will affect you, but everyone who you interact as well. Instead just focus your energy on praying for your mom to have the strength to endure.
To close, here's a quote from the movie "First Knight" I always whisper it before I make a life decision ;
May God grant me the wisdom to discover what is right, the will to choose it, and the strength to make it endure.