Why do I have to put up with their family?

You have a choice: you can either put up with the granddaughter, or you can set a boundary and risk the friendship.

But you cannot expect your friend to change without you setting a boundary and then standing by it. You may have to set up a meeting with your friend, and say, “if your granddaughter comes along, I will leave. I want to spend time with you, not with her.”

And then honor your word.

To find out what’s going on, your best bet would be talking about this with your friend.

Just off the top of my head…is your friend worried about this girl? Does she feel like she is not spending enough time with her? If that’s the case, I can see her bringing the kid along to soothe her conscience. Additionally, maybe she feels you’d be a good influence on her granddaughter.

If you don’t want to ask straight-up, I’d broach the subject of “granddaughter” and find out what is going on there. You absolutely should be able to spend time with your friend alone, of course.

Just wondering, but where is the daughter in all this?
Do the daughter/granddaughter live with her?
Pretty sure grandma is not just babysitting, not a HS junior, so there is a reason they are together.

Perhaps as @longgone said, maybe she feels that you, and perhaps herself, are a positive influence that this girl needs and is not getting elsewhere.

I am not sure I would go the route that @zenvelo suggests, setting ultimatums and such but it is certainly a topic that needs discussion if only for your own peace of mind.

You need to know why and, if you are good friends, there is no reason you cannot bring it up in conversation next time you call to arrange to get together.

Perhaps something along the lines of:

You: ”.....so can we get together this Wednesday?”
Her: “Sure that would be great!”
You: “Will your granddaughter be joining us?”
Her: “Possibly, I don’t know. Why?”
You: “Well, I was just wondering and trying to make plans accordingly. I know you like to bring her along sometimes. I was wondering why you include her in our get-togethers occasionally….?”

And see where it leads from there. No threats, no accusations, no ultimatums, and maybe the passing on of needed information.

A little off topic, but I have to wonder why the granddaughter goes with you all. It sounds as if she is very bored by it all. Surely she has friends she would rather hang with.
But my first thought on reading this is the grandmother wants to help her out by being with her.

I think a conversation more along the lines of “I’m wondering if we can make plans for a time when your granddaughter won’t be with you.” I think the @zenvelo route seems a bit harsh (“If your granddaughter comes along, I will leave.”).

Then when you see her, try to ascertain why she brings the granddaughter so frequently. Maybe the mother of the teen doesn’t want to leave her home alone too much. In that case, maybe they can arrange for the teen to spend time with her friends when you are going to see grandma.

I would think, for a teenager, hanging out with adults would probably be boring. Unless they’re getting rewarded by the grandma purchasing clothes or something for them, or maybe the granddaughter has a terribly boring life.

Thank you for all of the suggestions. I was really hoping for some ideas from others. A different vantage point and indirect involvement makes a big difference. I am too close to the situation.
My friend’s daughter is on the scene and involved. My thoughts are that the whole family talks a little too freely about adult topics, regardless of who overhears them. This girl attempts to insinuate herself into all conversations and makes very inappropriate statements. She has been badly spoiled, loves drama, and is very immature. I do NOT wish to be involved with the whole family.
I will open a conversation with my friend and use the suggestions I have received here. Thank you all for taking the time to respond. I appreciate it! :)

I’m wondering why you want to be friends with this person. She won’t see you alone and imposes her boorish granddaughter on you. Instead of giving you her full attention and some one-on-one time, she creates an uncomfortable triangle.

You’ve asked for suggestions, so here’s what I’d do—I would give the woman one final chance. I’d make it clear, in advance and within no uncertainty, that you want to get together without the granddaughter. Don’t express your dislike for the girl; that would achieve nothing except hurt feelings and anger. But, be adamant that you want your friend’s company without the granddaughter in tow. If your friend ignores you and drags her granddaughter along, take a break from this friendship. You don’t need to make it ugly; it’s easy to fade out of someone’s life by taking a softer, more passive approach (being “too busy” to get together or talk on the phone).

I know you care about your friend and would prefer to fix this problem. There’s a natural ebb and flow to relationships, however, and this might be an “off” time for the two of you. Backing away now doesn’t necessarily close the door forever. You mention that the granddaughter is a high school junior; the situation could mend itself soon, when the girl leaves for college, gets a job, or is otherwise preoccupied and unavailable.