What Hope Rainbow said is very applicable. Trijang Rinpoche did not really elaborate much on this incidence in his auto biography. It is more on how we interpretate his action.
I always have full faith of the actions of high attained lamas, such as Trijang Rinpoche, his not exposing or making it a loud affair of these two Sanga members who had cheated him must have a valid reason. Like what Hope Rainbow said, it only take one sanga member to behave badly and appears on the tabloid magazine, the whole world would be jumping up and down and start critizising, doubts would arise, respect would be lost and as a result, people will have less faith in the dharma.
Like the saying goes, one bad apple spoils the whole basket. So sometimes a high lama would just have to act and play dump, but that doesn't mean he is not aware of this at all. The main reason is just to protect the buddha teachings.
We must also understand that not every Sanga member is free from fault, they are also normal human beings who are subject to greed, anger and desire; if not they would already be a buddha. having said that, the negative karma these two sanga members created in cheating Trijang Rinpoche with the vows they had taken would be definitely much much more heavy.