In addition to what Big Uncle has highlighted from the 50 stanzas of Guru Devotion, the repercussions are surely very bad:-
(10) Having become the disciple of such a protecting (Guru), should you then despise him from your heart, you will reap continual suffering as if you had disparaged all the Buddhas.
(11) If you are so foolish as to despise your Guru, you will contract contagious diseases and those caused by harmful spirits. You will die(a horrible death) caused by demons, plagues or poison.
(12) You will be killed by (wicked) kings or fire, by poisonous snakes, water, witches or bandits, by harmful spirits or savages, and then be reborn in a hell.
(13) Never disturb your Guru’s mind. Should you be foolish and happen to do this, you will surely boil in hell.
By causing others to break their samaya (leaving and having bad thoughts about their guru), this person is also breaking his/her samaya and will receive the same repercussions.
If we can receive merit from encouraging others to dharma, we can also receive demerit from turning others away.
(16) (If from a lack of awareness you have shown disrespect) to your Guru, reverently present an offering to him and seek his forgiveness. Then in the future such harm as plagues will not befall you.
As a Buddha, a Guru will never hold a grudge, showing him disrespect cannot possibly offend or hurt him. The only one you harm is yourself. Therefore if you repent and beg his forgiveness, he will accept what you offer with great compassion. Then by the force of your faith, respect and devotion, you need not experience great misfortune.
The beneficial effects of Guru-devotion and the dire consequences of a breach of it are not rewards and punishments from a godly Guru. They follow directly from cause and effect. Your Guru is the focal point for your practices leading to Enlightenment. The more devoted you are towards the state of perfection he represents, the closer you come toward this goal. Despising him can only tale you further away into darkness and ignorance.
Point 16 from the 50 stanzas of Guru Devotion shows it is possible to seek for forgiveness. People who has done harm and caused others to leave have more chances to repent if they are still with their guru. How about those who has completely left and whose mind is less stabil? Will this person meet dharma in the future ever again?
Time to contemplate deeply before we cause schism and put other's happiness in jeopardy because of our selfish, unhappy mind.