In terms of what is going on in her relationship, I am just steering well clear of that. We are not going to be living here for much longer, and that's her mess to make. However, I *do* know and he is open with me about their relationship. So, yes, I agree, what she is doing is cheating and having an affair. He isn't.
To return to the texting, he is prepared to ignore it a little more at times, for example we went away for a weekend together and their texting was kept to a minumum of a couple in the morning and evening to say hello and goodnight. That was lovely for me. But on other occasions we'll be out watching the football at the pub together so won't be talking that much, and he'll have a text exchange of up to 20 or so texts (without exaggeration) and this gets extremely frustrating. It's good to know it's not just me who thinks it's rude!