I've been wondering for quite a while whether it shouldn't read "________ to Théoden" instead of other way round like you put it,
TM. Looking at the relationship between Pippin and Denethor, what have we got?
1.) Pippin told Denethor how his son died,
2.) swore fealty to him,
3.) served him as his squire and, I believe, cup-bearer (the
in-vino-veritas hint could refer to this),
4.) disobeyed his orders to save Faramir's life.
I can think of several people who performed one or more of these services for Théoden, but no instance of Théoden himself doing anything of the like.