Hey folks! Just popping in here real quick to confirm that your snake friend does NOT report to enemy players while you are stealthed via Mesmer nor Thief stealth. Like many effects (auras, footfalls, giant floating books) the winged snake does not appear to stealth to you, but no player effects report to enemy players while you are stealthed. I just confirmed this on our development branch to be sure I wasn't mistaken.
Enjoy your new legendary weapon!