TL/DR - but unless one of those birthday bitches wants to participate, I am pretty much the only married estrogen on this board... so...
I know this is not the crowd from whom I should expect any sort of consideration for the opposite viewpoint. But there is one - and read all the way through before you go off on me for being a stupid woman.
A man could be flat out falling down on the floor with exhaustion and would still be able to get it up and have sex. It's biology. Very little short of unconsciousness (I am allowing for age-related issues here) will prevent a man from having a hard on and that's pretty much all you need to have, other than a willing receptacle.
It is not like that for a woman. The biological reaction necessary for a woman's body to be ready for sex is a lot more complicated. If she is not physically ready for sex, it hurts, plain and simple, and can hurt for a while afterwards too. So being tired and mentally stressed out and whatnot are legitimate factors for turning women off sex. Guys generally don't get that, and that's understandable, as they have no basis for comparison because they are not hardwired that way. We don't understand the level of pain you feel when someone kicks you in the nuts either. Doesn't make it any less painful for you.
Token is right about her needing to feel sexy - but how the hell does a woman feel sexy if her man doesn't tell her that? (And get over yourselves if you think that means she has to look like a swimsuit model. I'm guessing Men's Fitness isn't knocking down doors either. A sexy woman is one that wants her man - end of discussion.) Not all guys know that you have to prime the pump long before you get into the bedroom - sweet gestures, kind words, notice her, etc. that make her feel sexy. Don't just roll over in bed and the first words you have spoken to her since you got home from work are "Hey, baby, feeling frisky?" Because 99% of the time, the answer is going to be an unqualified "NO." This was my ex-husband's MO - he would come home, actually ignore me totally and completely until lights out, and then expect me to be ready and willing. Um, no. You want instant sex with no consideration? Hire a hooker or buy a blow up doll. Women will shut down a sex life because we resent the hell out of our husbands for using our bodies to make themselves happy when they could give two shits about making that willing body happy in other ways at other times. It makes us feel no different from a set of golf clubs - take us out for a spin when you want some fun, then shove us back in the corner until the next time. That is very degrading and causes a lot of resentment. And let me tell you from experience that when a woman feels unsexy and resentful at home, and some other guy comes strolling along and makes her feel like the hottest thing on two legs? He might just get what you claim she never wants...
That being said:
If this were a woman I knew, my advice to her would be to remember that men like to fuck. Men NEED to fuck. They ARE hardwired THAT way. In this situation, her husband probably won't mind if getting it regularly means a fifteen minute quickie every now and again (+/-, your mileage may vary). It doesn't have to be long and drawn out every time, and honestly requires very little real action on her part. When you don't feel like it and he does, you have to weigh how much you love him and want to make him happy - especially since she admits that this is exactly what she did when they first got together - she loved him and wanted to make him happy. You cannot expect a man to love you and be faithful to you and deny him the one thing that makes him the most happy. Get some KY, and tell yourself that it is 15 minutes of your life that you can certainly spare, 15 minutes of happiness for the one person you profess to love above all else, and 15 minutes that will pay huge dividends in the strength of your marriage. The Corndog and I have a great sex life, but it's not a multi-hour, candlelight and champagne scene from a porno movie or romance novel every damn time, five nights a week either. There are occasions when we both want to, and then there are times when he wants to, and I can take it or leave it, but I agree because I know it makes him happy, 15 minutes, blah blah blah. He is happy, he knows I love him, and I can be back to reading my book inside of half an hour. He gets this consideration because he shows me and tells me all the time that he loves me, and thinks I am a hottie (and his opinion is all that matters). We both win.
It's a two way street.