Collies are so smart and keen to please, this should be fixable.
The thing is, she has to be very definite about her action, then she'll only have to do it once or twice.
When he next bites (at all), she needs to forcibly lie him flat down on the floor, on his side, holding his head to the floor, and in a big, loud, deep voice, say 'no'. The dog will be surprised and shocked. Then after a minute, she lets him up and makes her voice deliberately high and friendly and fusses him.
Repeat as needed. Worked very well with one of ours who was biting the others, in excitement/obsessive collie behaviour.
It sounds as though it's happening cos the dog is getting beyond excited - so a good long run before they begin, to run some of the nervous energy off should make it less likely.