well, I know that you probably aren't going to want to hear this, but what the heck, it won't be the first time that people on here haven't wanted to hear what I say, so I am sure you will take it or leave it....
If her husband doesn't know then "texting" would be the least of my worries. I would be asking him to stop all contact with her entirely. Absolutely under no circumstance would I be okay with my husband engaging in any kind of romantic communication with anyone who is in a relationship with someone that doesn't know their partner is conversing in that way. There is no good reason I have ever found that cheating is a good idea and forgivable. It ends in nothing but heart ache for everyone involved and prolongs problems that should just be dealt with and tied up before anyone moves on to texting another lover.
That being said, I had this complaint from my husband a couple of years ago. We now have it worked out that I don't text while I am with him during quality time. If we are just hanging out I will text a bit, but limit myself to something that needs taking care of now. I usually tell him what it is I need to take care of and then end it as soon as I can. The same when I am with any of my loves... if we are having quality time the phone goes on phone only and I only check it to see if there is a text in regards to my son, if he is not with me. This same rule applies when I spend quality time with him (although I don't need to check for babysitters during that time, obviously
). Sometimes this gets vague and I often am left with not one single second of the day to myself and have to check texts during quality time. I try my best to keep it minimal and get it done so the phone goes away.
I hate it when others are on their phone around me. Absolutely hate it. So I think that helps me keep it in perspective. I like to know what it is that is drawing them away from me and be a part of what they are doing, but if I don't know I just plain find it rude and inexcusable for the long haul.