[quote=@Spin The Wheel]
If he used breath of the wind then he'd have better odds, but he doesn't have much of a reason to do that right now so that sounds about right. It also 0robably wouldn't have helped much anyway. She might end up having to dig through his clothes a bit though.
[/quote]

Okay, I'll increase his odds of noticing her and stopping her from getting to his bag by 5% since it's under clothing. I wasn't thinking about that. However, it wouldn't deter a skilled thief. 
 
So, he has a 15% chance of stopping her initial attempt and a 20% chance of reacting quick enough to stop her if she succeeds. Therefore, the first roll will have to be 85 or more, and the second roll ([i]if needed[/i]) will have to be 80 or over. Here we go...

Noticing the Snake-woman: https://www.roleplayerguild.com/rolls/24736 
Catching the Snake-woman: n/a

[b]Results:[/b] With a roll of 86, Mougin notices the snake-girl's attempt to steal from him and narrowly avoids this from happening. Perhaps he heard his clothes starting to be cut and reacted quickly enough? The specifics are up to you, but it should be a close call.