Do I remain friendly with the parents of my son's best friend ?

My 16 yr old son came home very drunk/stoned after partying at his best friend's house, unsupervised and unknown by me. (also 16, and an only child) The parent's went out of state last Fri. leaving the kid totally on his own until this past Mon. I sent the mom a text cuz I was worried her son was alone (he wasn't, 40 kids there i learned later) & could be in trouble. (text read: " A " came home really drunk. Don't know if D. drove him." the mom didn't respond. After sitting up with my kid most of the pm cuz i was afraid of alcohol posioning, i left her a voice mail the next day giving facts. (40 kids, large amounts of beer/vodka/pot, worried about your son) 4 hours later, she called to say "D. said there was alcohol, but it was no big deal and he's fine". I was so happy he was ok, i simply said great. she questioned how bad off my son was. I told her. Clearly by her ?s' and tone of voice, she thought i was exaggerating. I feel they were negligent as parents and certainly not a good friend to me for making me wait for any communication. I've heard nothing from them since. She made it clear they "are not going to babysit D. & his friends any more and they trust him implicitly".
My inclination is to cut ties with them. My son certainly won't be alone with him or at their house. Time with that kid will be at school and places out of my control (groups) but all verified and he's grounded through thanksgiving. Thoughts, advice, suggestions?