An engineer on my team wants a four-day week after maternity leave. How have your managers made that work?

I run an engineering group and one of my leads comes back from maternity leave in September. She has asked for a four-day week for the first year, with the matching pay cut. I want to say yes. I did my daughter's first year full time with a fair amount of travel, and I wouldn't recommend it. The tradeoff is real, though. Her team depends on her for reviews, and if Fridays become the day nothing moves, the people who end up covering are mostly other parents. I don't have a version of this that feels fair to everyone yet. If you've been on either side of a reduced week, what made it work, and what made it quietly fall apart?