The section on Shields (CRB 277) specifically mentions this:
When you have a tower shield raised, you can use the Take Cover action
(page 471) to increase the circumstance bonus to AC to +4. This lasts
until the shield is no longer raised. If you would normally provide
lesser cover against an attack, having your tower shield raised
provides standard cover against it (and other creatures can Take Cover
as normal using the cover from your shield).
Here's a portion from the section on Cover (CRB 477):
Usually, the GM can quickly decide whether your target has cover. If
you’re uncertain or need to be more precise, draw a line from the
center of your space to the center of the target’s space. If that line
passes through any terrain or object that would block the effect, the
target has standard cover (or greater cover if the obstruction is
extreme or the target has Taken Cover). If the line passes through a
creature instead, the target has lesser cover.
This means that you are providing lesser cover to your friend by standing in front of them. On your turn, you Raise a Shield to gain the normal shield AC bonus. The first quoted section on shields states that having your tower shield raised increases the lesser cover that your friend is receiving from your positioning to standard cover (raising the +1 circumstance AC bonus to +2). In addition, your friend can now Take Cover behind you as long as your shield is raised.