That's exactly what he called her... "Mi Guera". And his family said it's not a flirt. It doesn't have the same flirty connotation as it does in American culture and not to worry.If I'm not mistaken the term "Guera" is Blondie for Mexicans when referring to a white blonde or semi blond female. This term does not have the same connotation in Mexican Spanish as it does in english: unless you are saying "mi guera" = my blondie, this is more than a term of endearment, it is outright a possessive term.
Nonetheless, when women start to trash their man like that to another man, we all know what's in store down the road.
I live in New Mexico and have a lot of Hispanic and Mexican family members and friends (I'm part Hispanic too). I have NEVER heard the world "blondie" used as a term for a friend. I have been called "blondie" by men who flirt with me because I am blond. It's a way to flirt, not a way one talks to a woman who is just a friend. His family fed you some nonsense. Do they tend to stand up for him no matter what he does?
Your huband was having what is called an emotional affair (EA) with "blondie". Here is a book that might help.
You have no reason to feel guilty for checking his texts/messages when he started to hide from you what he was doing. You have the right to protect yourself as you did.
When I did some snooping that resulted in my finding out that my husband was cheating, he told me that since I snooped, he would never be able to trust me. My reply was that he could always trust me. He could trust that I would also do what was needed to protect my children and myself. And if that means that I have to snoop because he's lying, then so be it.
If you can still get copies of those messages between him and blondie, I think you should make copies. If you ever find out that he is communicating again with "blondie", you should send copies of all the messages/texts to her husband. That should put an end to her flirting with your husband.
What you found them doing in messages is how affairs start... when people start complaining about their spouse and seeking sympathy/support from someone of the opposite sex, it's the start of an affair.