Tuning question

[Brion Toss]

Hi there,
That's an unusual configuration. As you say, this will tend to impose bend, but not, I think, as much as a babystay would, given the larger angle of the babystay to the mast. If lower shrouds are "swept" fore and/or aft, it is to allow some degree of fore-and-aft staying, in addition to their primary role of lateral staying. But imagine that this rig had only aft-led lowers; in the absence of swept spreaders, or some other geometry to oppose the pull of the lowers, the mast would tend to invert as the shrouds were tightened. In the same way, forward-only lowers will tend to pull the mast forward, but most likely leave it more vulnerable to pumping, especially when hitting chop.
But it could get even worse. Depending on the amount of sweep, any forward bend might only be accomplished by excessive tensioning of the lowers, because the forward component of pull is too small. This will make for a lousy lateral tune, undoing any gains made in fore-and-aft mast shape.
I might be sounding curmudgeonly here, but in the absence of compelling evidence to the contrary I'd say your friend's boat is misconfigured, and that the best thing would be to tune as for athwartship lowers, leaving any fore-and-aft bend to the backstay, ideally in combination with a hefty masthead crane.
Fair leads,
Brion Toss