I have the BIIX in both a 4 wt. and 6 wt. and, for me at least, I think it's pretty much the ideal action. That being said, I've read varied opinions from owners of that rod as to what constitutes the "ideal" line for them. I've read plenty who think it's a rod that is perfectly suited to a true-to-weight line. I'm one of those who like it better with an overweighted line, like the GPX or the old Rio Grand. I always liked the Rio Grand even better than the GPX, but they've apparently changed that line to a full weight heavier now instead of the old half weight heavier that I've enjoyed. For what it's worth, it looks like the Rio Avid line now features the same taper profile and weight characteristics of the "old" Grand. Either way, I think your friend might be best off spending a few minutes with you or some others, stringing up their 5 wt. lines and seeing which best suits his casting style.