Buddhist here - very sorry this has happened to you OP. As with any spiritual path/system, people can (and will) stuff up. I come from a Theravadan Buddhist background, more so than Tibetan (not that this should matter!).
From his perspective, he probably should try (as subjectively as possible) to examine his thought/emotional processes as this happened (as rationally as possible) and see if there is any way to see how these processes/conditions arose, so that he can identify them and catch himself to prevent similar things happening in the future (this can apply to anything).
Also, he might consider what take actions he can take to alleviate the damage done (if possible).
There will also need to be a hefty amount of forgiving himself and asking forgiveness of others (otherwise everyone involved gets dragged into a downward spiral of negativity - which is what it sounds like he is doing).
Mindfulness is key, it really it the foundation of all Buddhist practice but it takes a lot of work off the cushion as well as on it. There are four aspects of it, getting more and more complex: (1) body; (2) 'feeling' (attraction, repulsion, neutral towards objects/situations/people); (3) mental processes (thought processes); (4) phenomena (this is more complex can't do it justice here...).
Right effort is also important, the gist is: 1. to prevent the arising of unarisen unwholesome states, 2. to abandon unwholesome states that have already arisen, 3. to arouse wholesome states that have not yet arisen, and 4. to maintain and perfect wholesome states already arisen.
Hope that helps, if not, I can have another go at throwing words together.
I hope things work out for you OP.