widdly, when i read this thread i got goosebumps! u have described ALL of my DH's friends relationships!! Cheating and violence is often taboo talk so its a great step that you are aware of it and can make informed decisions regarding your DS's interaction with her DS. Remember that people from all walks and ways of life come into our life for a reason. Perhaps the reason she came into yours was to show you how resilient and strong you are, and this is only just early days. You never know, the friendship the two DS's develop could be a lifelong and fulfilling.
And DITTO to Slyder! Some people just love drama in their life!