I personally don't think your friend should worry about it just yet.
What if he didn't buy the ring? He could have possibly just been looking to see what some places had to offer and bookmarked a place he thought was interesting.
It's always better to go to the store (and take a good friend of the bride-to-be [that can keep a secret]to make sure you get something she likes)-- to make sure the diamond is perfect
I don't really think it matters how much he spends.. but that's just my opinion.