Wearing a Friend's Tefillin
Question
If someone wore the tefillin of his friend or father, is he obligated to later put on his own Tefillin when he can (after the prayer, but not necessarily immediately), or is it just preferable? (And what about a yeshiva student who might have to put them on in front of others and feels uncomfortable being seen putting on tefillin in the middle of the day)
Answer
Shalom Rav
One who wears the tefillin of his friend, has fulfilled his obligation Le'chatchillah and does not need to put on his own tefillin again.
However, he must ensure that the tefillin belongs to a God-fearing person so we can assume that the tefillin are kosher. Additionally, they should fit the size of his head.
Source
See Mishnah Berurah, Siman 110.