Google "Startup Lawyer If I launched a startup 2014", which sums up most legal questions including equity splits. Founders Dilemma by Wasserman covers the topic from a scientific research point of view.
tl;dr as far as I remember from that book: Make sure it is fair, and enough equity for both to be motivated. You have to discuss with each other to find out. It can be everything between 95/5 to 50/50. If you value the friendship more than you value the company, you should go for 50/50. Otherwise 50/50 is not a good option according to Wasserman's research.
5% would be absurdly predatory for essentially a cofounder. (If they're building the company from the ground up rather than merely shaping it, then they're a cofounder.)
tl;dr as far as I remember from that book: Make sure it is fair, and enough equity for both to be motivated. You have to discuss with each other to find out. It can be everything between 95/5 to 50/50. If you value the friendship more than you value the company, you should go for 50/50. Otherwise 50/50 is not a good option according to Wasserman's research.