50/50 is a really, really bad option for the kids IME. It's mostly about assuaging the guilt or massaging the ego of one or both of the parents and embracing the model imposed by the family courts in a failed attempt to be equitable and fair (to the parents, not the kids).
My DD, who is 9, really does not like having two homes. Kids want their own rooms, their own stuff, their own routines. They want to feel comfortable. They do not want to be ferried around from pillar to post every 5 minutes because Daddy thinks he's a better parent.
I am speaking from experience as a 75/25 non-resident mother. I am not going for a higher percentage of custody because the kids don't want the messing (even though their father is a performance parent only, and will not do the grunt work which means their hygiene, clothing, phone usage etc. is atrocious with him. Hopefully not your friend's case).
He can still be an important influence on them if he sees them twice a week. My dad was a far greater and more positive influence on me than my mum, and I lived with her full time. What is important is not how much contact a parent has, but the quality and consistency of it. It must be regular, but relaxed and non-stressy. Otherwise the kids get upset too, and your friend will have years of animosity and vicious text messages with the ex, taking precious energy away from parenting and ruining the children's memories.
I say this as a child of divorced parents, I absolutely detested having to "move house" every weekend (and that was without the midweek faffing, which I didn't have).
Can your friend put his ego aside and accept that if the mother is allowed to do her job properly, she will probably be far less anxious and easier to deal with? I find the assumption that she "can't cope" to be patronising, what is it based on? His perceptions, and...? She might be anxious because she has to deal with HIM.
If not, if your friend has serious grounds to state he is the more suitable primary carer, then go for a less stressy option where they live predominantly with him and see her regularly.