Hello!

Referrer links are a good way to go (and very common) but there is a "cleaner" way to do it if your friends have websites...

Google Analytics has something called Attribution Modeling that you can use to track where each conversion (or sale) actually comes from.

You can decide to set it up for "last interaction", "first interaction", etc depending on your particular model.

This means that either the first or last person to refer your client gets the credit.

It is worth checking out and it's free!

Let me know if it works out.