In other words, BGT is worth it even if you only play it for BG2+.
BGT doesn't add anything to BG2, if you are only playing BG2.
Not entirely true. BGT tweaks a number of things behind the scenes. It adds some VERY helpfull map notes that reduce note taking and NPC locating time. And, have you ever done a total conversion of an older game you liked and NOT re-played it. If he upgrades now, before he gets too far into BG2 then he won't have wasted as much time, he will have BGT sitting there begging to be played in it's entirety and if he ends up not re-playing BG1 in all it's glory then he will have wasted a little time but will have the entire experience (minus any other mods he may want) already loaded and ready for future replays. Is there some reason you are against BGT.