Should you buy the actual ring before popping the question? One of my friend actually ordered a set of 3 stackable rings at the jewellers but broke up after giving the first promise ring...?

I think you should start by asking yourself the question: is there a reason why your friend is spending money on the ring instead of the other way around? Why should you friend be left bagholding instead of the other person?

It is time your friend sent their ex a bill for the rings.

Urmmm I think it depends how long is the relationship and whether if both parties are serious to commit to marriage. Since most people will only find out the little yet kinda abit dislike things like toothpaste (press from bottom or press at the middle), time will tell if the couple is suited for each other.

So.... just wait until the time is right (maybe 2-3 years?)