What would you do?

Some guy followed me on Instagram the other day, I looked through his profile and deemed him harmless so I followed back. In his profile bio he mentioned his career, children, and girlfriend. This morning, he messaged me, saying where he had found my username (on a site that's commonly used for dating, though that isn't its only function) and said something to the effect of "I just couldn't help but notice how good looking you were." I stated that I was interested in flirting, to which he questioned why I was on the site. I simply said I use it for entertainment, and asked why he wanted to message me. He said something along the lines of "I thought that's what people did but I guess not," which didn't make much sense and was incredibly vague. So I told him that led me to believe his intentions were likely less than decent, considering he was so defensive. He denied it, followed with a "but whatever." I asked if his girlfriend knew that he was messaging women and commenting on their looks. He read the message and blocked me right after, therefore I feel it's safe to assume she doesn't.
Since he made mention of her in his bio, I looked at her profile. She posts about how great he is frequently, likely oblivious to his behavior online. As someone who was cheated on - and in such an "obvious" way - this saddens me a little.
I know it's not really any of my business, but should I let her know about this? If it were me, I know I'd appreciate it. It may hurt in the moment, but I'd be glad that I didn't waste my life with a guy who can't keep himself from hitting on women online.