@bishopjordan I understand that you want to be protective, and love your son, but since he is 18, the only thing you can do is voice your concerns to them and give them guidance. Contacting her parents would come off overbearing, and you would probably end up just pushing them away. You should not be interfering in your adult child's relationship, even if you think it is toxic, because ultimately - it will cause you to look like the bad guy, and it often has a solidifying effect. It often causes a 'I will stay in this relationship because my parents don't want me too / stick it to the man' effect.
18 is a volatile age, and I know you wish he could focus on school, but it doesn't usually happen that way. Relationships happen, and with them - some of those toxic behaviors you mentioned. Hopefully, things will work out - and if not, it will be a learning experience. Being positive will make him open up to you more, but be careful not to judge, take sides or offer up too much relationship advice. Be there for him if he falls.
On another note, they can't just 'get' an apartment. In most places, they would have to have stable jobs, proving income 2-3x the amount of the apartment, with a deposit/last months rent to even be considered for one - so it would take them some time.