Get a job offer with a $hard_salary_number. Tell your boss you got an offer with another company, but would consider staying for $hard_salary_number * 1.15. If not, leave and go work with your old friend.
If you actually just want the new job, no reason to stay. But if you really really really like your current job, it doesn't hurt to get the new offer and then ask for a raise. If the raise doesn't happen, leave.