Why is it a problem to you when they are both adults? Maybe the problem you should really have is when he cries and complains about these relationships- maybe that’s the real boundary here you need to draw is that you won’t be his cry shoulder anymore. But breaking a friendship just because the man is into younger adult women who are willingly in a relationship with him doesn’t make sense since it’s not your life. It’s not really your place to make judgments about him!! I am 27f and I have dated 40+ year olds before the reason it doesn’t last is the difference in opinions and lifestyles- I learnt my lesson - he will learn his in due time. You don’t have to pressure him into learning his lesson unless he’s doing something illegal.