You wait for them to finish. Then feed them water and crackers.
Like really you should wait cause the body is cleaning itself of what is essentially poison.
but if you can't for what ever reason:
Weed sometimes works i've heard but I've only ever seen it make it worse in person.
but high does benzos.
Lorzas or xannies double maybe tripple dose, should kill the vomiting.
DISFUCKINGCLAIMER: Mixing high does perscription meds with alcohol is dangerous, DO NOT do this without consulting a doctor first. /don't sue me if he dies.