How can I arm a friendly peasant?

Unfortunately, there is no way to currently arm companions -- they're stuck with whatever they're generated with.

The sole exception is if they wrestle in combat, and manage to gain possession of an enemy weapon, either from reclaiming a weapon lodged in one of their wounds, or by grabbing the enemy's weapon directly.

However, both situations occur rarely, and cannot be relied upon because your companions don't use any kind of strategy in their attacks, or at least, not a strategy that you can suggest them to use.

This can actually be achieved though some mods. one mod that lets you easily do this is dfhack. after installing it properly go into adventure mode and use the look command at your companion and view their status. after than go into the dfhack window and type this command in without the "": "adv_bodyswap" then pick up the items and equip them as the companion. after that inspect your adventurer and type in the command again.

this command can also be used to "hijack" other creatures bodys by ading the command: "force" at the end. "adv_bodyswap force"