5% is a good number I've seen a few times in the same circumstances. If this were pre-funding you should be asking for a solid third of all equity since you are a cofounder.
For me, 5% sounds more like a the offers I've gotten as first Eng hire, rather than co-founder. A couple months ago, "cofounder" should have gotten this person around 1/3. Now, I think it's reasonable to look around half that - 10-20%, depending on cash comp versus market.