There are so many thoughts that come to mind in regard to your question. It's difficult to know where to start.
In general, recital of Tehillim have the characteristic of removing anything which blocks or obstructs blessings from HaShem.
So in that context, don't think that your recital of any particular chapter is somehow failing to help your friend.
In that context, a story from the Ba'al Shem Tov comes to mind.
There was a Jewish indidual who had been conscripted to the military when he was a child. He never learned how to practice Judaism properly in its details. But he knew and connected with being Jewish.
At a particular time, circumstance arranged that he would be present in the shul of the Ba'al Shem Tov for Yom Kippur. This young man didn't know Hebrew or which prayers to say.
The story can be seen at the following link...
There are many details which are, maybe, more efficacious, but the primary thing is your personal intention. If that is positive, G-d and the Angels will find a way to make your prayers effective.
G-d bless you