If you know when it happens, you'll need to be there and see if that is where it's coming from ?
Does the roof-jack extend above the peak of your roof ?
At 30 years old, it may be a seperation in roof-jack.
Did your friend replace those sections with Coleman roof-jack sections, crown, extensions, etc. ?
It is a must to know where the draft is coming from before you can begin to find a resolution.
Some people are Humbly Grateful, while some are Grumbly Hateful.................... Which one are you ?