Abba: “You will also need to have a good relationship with the ex wife as she can make your boyfriend’s life and if you marry him you life as well, a living hell, as long as she has custody of his kids.”
This is a very good point. Even though of course you have to take the guy’s side (if you marry him), you also want to make sure to maintain a certain amount of neutrality in terms of how you relate to the ex-wife. Being antagonistc to her will hurt your relationship with the kids and b’ezras Hashem,the future grandchildren whom you will be sharing with the ex-wife.
The second wife can actually be helpful in terms of neutralizing the situation and leading to a more peaceful situation in which the two sides are able to communicate more peacefully and civilly, which is important as B’ezras Hashem, there will be shared Simchas in the future. I have seen this happen.