I agree with what's been said so far. You need to talk to her. Tell her you have different goals and would like to work out some rides to do with her, but that you will be doing other rides on her own. There's nothing you can do about how she will respond. If she is truly a good friend, she will understand, even if it hurts a little. If she gets mad at you, maybe she's not such a good friend after all.

When my much stronger husband rides with me (and usually my friend), he usually pulls the entire time. This gives him more of a workout (even if not as much as he would get with a faster group or alone) and helps us ride faster than we might have ridden. Does your friend draft?

Sometimes my husband will choose to ride with us when wants to do a recovery ride. And, like others have mentioned, he'll ride hard up a hill and wait for us, or put it in his big ring and pedal in a super slow cadence which works his muscles differently. He finds other ways to challenge himself. However, most of his rides are with his faster riding buddies, and that's totally fine with me. Luckily, I am blessed with a riding parter and good friend who I am very compatible with. She is much faster than me on the hills, unless it's a short climb that I can attack or fairly low grade. This drives me crazy, of course, but it also pushes me to work harder. I am a stronger rider because of it. I have had to let go of the frustration of always being the last one up the hill. I am a faster climber than a lot of people, just not the people I most often ride with.

Like I said, if she is a good friend and you are sensitive in how you present yourself, she will understand. If she doesn't, you should maybe look for a new friend anyway.