Easiest thing to do is just take the snake to the vet for a shot. They'll shoot the snake up with something close to ivermectin which will have to be repeated 2 weeks later...(usually they just give you the syringe for the second shot if you know how). Very effective. Probably much cheaper than prevent-a-mite in the long run.
It works basically like top spot for dogs. Any mites that bite the snake dies. Two weeks later the shot is repeated to kill the mites that have just hatched....it might work with inernal parasites as well.
I hope that this person either does't have a big collection or is a big believer in quarentine. What ever you use, do not assume the mites are gone until you haven't seen one for about 4 months....I know that is extreme, but trust me on it. I once bought a female boa at a really good deal. That $100 snake ended up costing me about 2K before it was all said and done. Now ALL animals I buy are quarentined for 1 year.
Travis Rose
P.S. If they have a large collection, tell them to talk to the vet. You can actually take ivermectin and do a serial dilution with it to make a cage spray. It is super super dilute and will not harm the snake. I've done it in the past under vet guidance.
Currently
0.2 Normal Columbians (7 & 9ft)
0.1 66% Snow Columbian (6ft)
0.1 Het Anery (4ft)
1.0 DH Snow (5ft)
1.0 Salmon 66% DH Snow (2.5 ft)
2.4 Kenyan Sandboas (6-30 inches)
Hopefully more on the way.