Short answer: If current value is easy to prove, check that box quickly and focus on future potential value and his plan to get there.
Current value is more important in a seed round than future value potential. Proving that you can execute on a product, achieve market fit, and position it to grow are all more important than proving the business could be huge. After all, if you can't do the first three, how huge the business could be is irrelevant because you'd never get there.
That said, if your friend's resume and the metrics of the business are such that it's relatively easy to prove this, I would give it little emphasis. Dwelling too much on how well things have already gone can only do bad things like:
a) invite scrutiny of existing operations to the detriment of discussing future plans, which are more relevant to your planned fundraising.
b) send the message that you think you've already achieved success and are looking to be rewarded for a job well done.
If, on the other hand, it's not easy to prove this, it needs the emphasis because, as I said, proving that is far more important than demonstrating a huge hypothetical opportunity exists.
As for what constitutes "proof," growing users and improving retention is great. If acquisition and other variable costs are flat or falling, I'd say that's good enough. Absent any of one those things, solid proof you can get there could suffice. E.g. a detailed plan, a successful track record executing in similar situations, etc. But those have to be stellar.
Just one man's opinion.