To avoid cases like this you can adjust your Fraud settings.
1. Limit the # of Advocate Referral Incentive rewards each Advocate can get in 30 days
2. Limit the # of Advocate Referral Incentive rewards each Advocate can get in total
3. Limit the # of Friend Incentive rewards that can be issued from the single Advocate Offer (note, that means from a unique link from a single campaign).