How does your friend conclude that being "single" is going to improve your their financial situation? That seems to fly directly in the face of your prior statement that both parents make "6 figure salaries" and that "income isn't an issue". If he wants a better financial situation then he should explore if there are tax advantages to being actually married. There likely are. Two adults (one with a child) are rather unlikely to see an an improved cash flow by living in two homes. Unless of course your friend hopes to just ditch his financial responsibilities...
In my opinion, Your friend needs to "man-up" and accept his responsibilities particularly with respect to being a father. Likely their partner does as well. That almost assuredly means re-prioritizing the family and particularly the child over things that had top priority before the child was born.
As a father, I know the initial sting that comes with a loss of a prior life and the changes in family dynamics following the birth of your first child. I can assure your friend that many of the aspects that make toddlers a challenge diminish from this point forward.
Your friend stands on the edge of a terrible decision, not just for their child but for themselves as well. The joy of being a parent is more than equal to the responsibilities. Your friend and their partner loved each other in the not too distant past. If they reflect on the things that brought them together in the first place (probably with counseling) and make the life changes necessary to prioritize their family then I feel like they have a chance of rebuilding that love.