[RPG] Can Saving Shield and Bodyguard stack

Apart from the traits you listed, there are some more that help with Aid Another checks listed on d20srd.

• Helpful is strictly better than Helpful (+4 vs. +3), with the downside of being raised by Halflings (via Adopted).

• Fools for Friends from Second Darkness gives a straight +1 trait bonus to whatever your Aid Another boosts. It stacks with Helpful (since the latter does not grant any named bonuses), But not with any trait bonuses your Ally has, which there aren't many to AC.

• Inheritor grants a +1 trait bonus to an adjacent ally, but you need to be a Gillman, and the same restrictions as above still apply.

The answer is no, the creature is not provoking an attack of opportunity--Seize the Moment is merely providing an AoO for something you normally wouldn't get. Contrast this with the wording of Outflank (taken from pfsrd)

Outflank (Combat, Teamwork) You look for every edge when flanking an enemy.

Prerequisite: Base attack bonus +4.

Benefit: Whenever you and an ally who also has this feat are flanking the same creature, your flanking bonus on attack rolls increases to +4. In addition, whenever you score a critical hit against the flanked creature, it provokes an attack of opportunity from your ally.

The wording of these two feats is very different, and you can see via the italicized portion that if seize the moment were intended to work the way your friend describes it would also have to be worded this way.