You need line of effect.
From the RC on targets, p107:
Line of Effect
... Unless otherwise noted, there must be line of effect between the origin square of an effect and its intended target for that target to be affected. ...
The only difference between effects that target allies as opposed to foes or creatures is that allies can ignore the effect if they want.
From the RC on targets, p106:
Ally means a companion of the power's user. When a power defines a target as an ally, the ally is free to ignore the power's effects. ... In contrast, a creature targeted by an ally's power cannot ignore its effect if the power targets a "creature" or "creatures," instead of an "ally" or "allies".
There are some differences between effects that explicitly target allies and those that don't, but they have nothing to do with line of effect. A power's origin must have line of effect to its targets, regardless of whether they are allies are foes, unless the power explicitly waives that requirement.