@Deryn I'm not sure I can be of much help -- but I'm reasonably certain that if you add much liquid "flavoring" to the custard you'll get a hard, icy result.
If I were to try pineapple orange as a flavor, I would pressure infuse chunks of cooked pineapple and fresh orange with cream in an iSi. Then strain the flavored cream. But to me pineapple orange sounds appetizing as sherbet, not so much as ice cream.
Nonetheless I understand your friend wants ice cream. You might check with her if what she has in mind is custard based or not.
Another possibility if she does not want custard: macerate mashed cooked pineapple and fresh orange in sweetened cream and freeze. I used to do this quite successfully with peaches to make a Philadelphia ice cream.
Edit: I grew up in Philadelphia.
