As I mention here: https://news.ycombinator.com/item?id=30612983, at the monetary level, I don't think this is a conflict over a 50% stake, but rather over the valuation of a 45% stake (at the end it would be a 95-5 split).

But in general yes, my impression is there are usually agreements in place to reduce an ex-founder's share of the company over time.