Be honest.
Tell her something like: "I hate being an asshole, but i hate being dishonest even more"
The direct approach isn't always the nicest solution, but yer better off being honest and crush her feelings right now, than lying your ass off and hurt her even more by toying arround with her feelings for weeks/months/years.
If she truely is a good friend, she'll understand your opinion and value your honesty.
She might not like it, but at least she'll know the truth and will be able to get on with her life.
On the other hand, if she is a smart girl with a sense of humour you could always choose not to be a fcking mainstream retard who only wants the perfect figure and enjoy the fun as the people you are, instead of seeing every girl as a potention fckdoll and demand more then actually needed.
And yes, i've had women who wheren't exactly stunningly beautiful to look at, but at least they where able to best me in a contest of wit, rather then giving me the "Duhhhhhhh"-look with every joke i made; not to mention they are usually better in bed than the ones with socially-demanded physicial features.
And being the bold dick that i am i can safely say that i rather have a smart and ugly girl who knows what to do, than a good looking novelty that doesn't know sh*t, let alone even tries to do so.