I'm sorry your son was bitten and I hope he is feeling better soon.
But IMO the problem is not terriers (either a specific one or in general ) but your friends attitude towards training her dog. She didn't / doesn't bother to train her dog , she accepts his behavior as appropriate or just "terrier" behavior and lets him continue to act that way without any discipline ... so really what is the dog going to do? In reality most
trained terriers of anykind ,be it Yorkie or mix , do not act that way .
When I bought Sydney ( my first terrier mix ) I was told by my vet that terriers were a breed all their own
they were strong willed and high energy ,
but if properly trained they were a wonderful, loving , fun dog . She made it
very clear if I didn't want to take the time to train my dog , especially given the fact it was a terrier , then don't buy it. And believe me my little Syd Vicious needed LOTS of training!
So while I can see why you are upset (I would be LIVID also !!) Dont take it out on all terriers...yes ,they are a breed unto their own but with proper training they are wonderful pets ,but if left to their own devices and allowed full control , they will take it.
I mean honestly , would she allow a large breed dog to act that way without trying to regain control? While a bite from a big dog would certainly do more harm , a small dog needs just as much training and decipline as a big dog.