Do all fruit trees share a common fruiting ancestor? Or did some lines evolve fruit independently?
Do you mean seed plants, or Spermatophyte? Or maybe flowing plants-- Angiosperms.
All flowering plants share a common ancestor that was a flowering plant, however many of them don’t produce anything that fits the common man’s definition of ‘fruit’ (i.e. not the definition a botanist would use in the context of his profession).
But different species produce their fruits in different ways, from different parts of the plant anatomy; I think this is a fairly good indicator that edible fruits for example, must have evolved independently several times.
It depends entirely on how you define ‘fruit’ and ‘fruit tree’.
There are plenty of trees like plams that produce fruit yet have recent ancestors that would have produced dry hard seeds and seed cases. But those ancestors still produced seeds.
Most of our north temperate climate fruits are in the rose family (apple, pear, cherry, peach, plum, nectarine,…) but it also includes almonds, which don’t fulfill the layman’s definition of fruit. Wild roses do produce fruit (rosehips).
Even within the rose family though, there are different kinds of fruit developing from different anatomical structures, as well as many genera/species that produce dry or inedible fruit bodies that we. the general public, would not intuitively describe as ‘fruit’.
It may be that these all developed from a single ‘fruiting’ ancestor - i.e. the anatomy that makes an apple ‘fruity’ and the anatomy that makes a plum fruity, while different, may be specialisations derived from an ancestor that had both parts - but it’s quite possible in cases like this that they are both novel adaptations, arising independently.
Seed dispersal enjoys a relatively good deal of selective pressure; it’s not suprising that lots of plants have evolved strategies to augment it and it’s not surprising that many of these strategies happen to be similar.