Guiding Bolt grants advantage to the next attack against the same target:
the next attack roll made against this target before the end of your next turn has advantage
If one of my allies got hit with a Guiding Bolt, could I have my Str 8 (−1) Warlock punch him to use up the advantage granted to the next attack against him? Even if I hit, I believe a punch only does 1 damage, so my −1 Str would make that zero. Even a crit would only do 1 point.
I would think this would work since the description only says it has to be an attack. And a sneaky DM could do the same, by having a henchmen hit the Big Bad to use up the advantage. I mean, getting punched would be better than getting hit with a Greatsword.