There's a difference between grinning and bearing a puce bridesmaid dress and this...
I know when it comes to my writing (or other things) that I'm VERY nervous and when I ask someone to read it (taste it, view it, etc) it's in the hopes of getting positive reinforcement for my very self-conscious effort. Even when I say "Be honest" I'm really meaning "PLEASE at least find ONE thing I did right!" I try and brace myself for bad news, but still hold out hope that at least one thing (one sentence, one ingredient, one something) is good. That's why when I'm asking for opinions on a cheesecake I do NOT ask my FIL... he doesn't eat cheesecake. He doesn't eat cheese period. It's a consistency thing. No way could he offer constructive criticism because he hates ALL cheesecake. They ALL taste like (or rather feel like) crap to him. And knowing that, I would never put him in the position of having to choke down something he doesn't like and then try to do it with a smile and any kind of critique; constructive or otherwise. That's just not cool to do to someone you care about. Maybe an enemy... and you'd certainly hear about any bad points then... but NOT someone you care for.
It's HARD to be an editor. Not only all the grammar and what not, but also having to walk a delicate line between brutal honesty to create a book that will sell and not destroying the author so they just quit. Someone who has an emotional investment (their own work, a friend's, etc) is just going to have an even harder time of it.
But hey, let's say she does read it and LOVES it... it could happen. Is her friend really going to believe it? Knowing her likes and dislikes, is she going to believe her good friend is really being honest or is she just going to figure she hated it and is just being nice? She'd be right back to where she was before she gave it to her to read. That's why editors are great. When they tell you it's good you know they mean "This will sell"... because if it doesn't it's their bum on the line. Asking a friend... no way to know if they're being kind and honest or kind and dishonest.
That's why I'd be honest up front. Tell her you do not like these sorts of stories. That it'll be hard for it to hold your interest even if it's a bestseller (maybe mention an author of that genre that you've tried reading but failed??). That you're prejudiced against it to start and so wouldn't be able to give it a fair critique. That you'd hate to end up lying and saying you liked it because you know that wouldn't be ANY help at all. And that's what she needs, that's what she's asked you for.
