As always, your friend needs to find a divorce lawyer in his state preferably before he does anything else to ask these types of questions to. Then, he needs to think long and hard about what he really wants and what is best for the kids and avoid spite (easier said than done).
It depends on the state they are in, the judge that hears the case, and her person willingness to fight over it. There are different types of custody as well. She shouldn’t be able to get true full custody unless he has done something terrible and poses a danger to the kids. That type of custody allows both parents legal rights to make decisions for the children and see them on a regular basis. Full custody only allows one parent to make those types of decisions and he doesn’t want that.
However, there is also the question of physical custody. That mainly determines where the kids live. The father and mother can share joint legal custody but she could also end up with sole physical custody. That is pretty common but it doesn’t have to be that way. Some parents share joint physical custody as well and the kids go back and forth between mom and dad’s house for whatever they agree on.
If they are civil enough, they could work out their own agreement and have their lawyers write it and proofread it. It doesn’t have to be "every other weekend and “two weeks in the summer” for dad. I have my kids for major parts of five days a week and sometimes more because my ex-wife works late. I had that written into the divorce agreement and it is binding even though she technically has sole physical custody.