with the new exchange the service showed the wallet the same as last time
Do you mean they gave you the same deposit address for two different mixing orders!
Addresses reuse is not recommended for privacy reasons and if done by a mixer, which is supposed to enhance user's privacy, then this is a big red flag.
Maybe it wasn't a clear indicator that they were going to scam but it shows how bad their mixing service is.
But, you can put the alleged evidence in this thread, for all to see, so to speak.
I wouldn't recommend doing this, for privacy reasons (which is the aim of using a mixer in first place). But, if really needed by the community, he can send the letter of guarantee to one of the highly trusted members of this forum to check it and ask him to post here to confirm or deny this story.
I suggest LoyceV for this as he has done it before.