Mocha Stout Bottle Bombs!

This is my first post (the exploding bottle issue was on my mind when I created my account, hence the name).

Anyway, a friend of mine brought over a few bottles of mocha stout he brewed from an extract kit. Talk about over-carbed! Last night, two of the bottles exploded all over my kitchen. Huge mess and a pain in my ass.

I know this may be hard to answer without the recipe, but:

Would the use of unsweetened chocolate during fermentation possibly cause fermentation to last longer than expected? He let this beer sit in primary for 3 weeks and secondary for 3 weeks before bottling, and then used the priming sugar when he bottled (whatever the standard amount is for a kit for 5 gallons).

I dopn't know, that sounds like a good amount of time in fermentation, but he should have checked to make sure fermentation was done before bottling. the only other thing I can think of, if fermentation was complete, is WAY too much priming sugar... or some sort of infection during bottling.