No one can pray istikhaarah on behalf of another

Praise
be to Allah.

It
is not permissible for anyone to pray istikhaarah on behalf of someone
else.

It
says in Liqa’ al-Baab al-Maftooh, by Shaykh Ibn ‘Uthaymeen (no. 89):
Istikhaarah can only be done by the person who wants to do something or is
thinking of doing it. It is not valid to pray istikhaarah on behalf of
another person, even if he delegates him to do that and says: Pray
istikhaarah to Allah for me (and ask Him to guide me concerning this
matter), because the Messenger (blessings and peace of Allah be upon him)
said: “‘If any one of you is deliberating about a decision he has to make,
then let him pray two rak‘ahs…” By the same token, if two people enter the
mosque and one of them says to the other: Pray two rak‘ahs for me to ‘greet
the mosque’ (tahiyyat al-masjid), and I am going to sit down, that is
not valid. Istikhaarah prayer is connected to the person who is seeking
guidance concerning the thing he wants to do. End quote. 

And
Allah knows best.