Can I use heavy cream instead of creme fraiche? If so, the same quantity? Thanks

You'd also wind up with a slightly wetter batter, which would change how the fruit distributes, but I think it would still bake up just fine. As DebS says, you could use a mix of cream and sour cream or yogurt.

You will lose some of the tang that creme fraiche provides but yes you can sub in heavy cream in equal amounts. If you still want that bit of tang I would use mostly heavy cream with a touch of sour cream, buttermilk or even yogurt.