On what media do you burn? Some discs will be less compatible than others.
Also if you burn +R (?), it's possible that your friend's player doesn't play +R if it's a rather old player.
And no, there will be no difference if you burn and ISO. The resulting DVD is exactly the same.
What impacts the readability/compatibility of your discs is the burning quality, and the quality of the media (mechanical properties, reflectivity..). The burning quality depends on the combination: burner/firmware/media/burning speed.
So you'll get more help if you give some more info about your burner, firmware, exact discs brand/model and burning speed.