Mommy Advice Needed

Let me set the stage for you:

My daughter is 7 1/2.  She is very independent (most of the time) and responsible.  She is also very trustworthy.  Unfortunately, she has no siblings yet.  We are working towards fostering to adopt.  We don't have many neighbor kids that "come out to play".  Some of them have moved and some have siblings that they really just play with. 

The girl who lives next door is 9 or just might have turned 10.  She always has her little brother with her who is 4ish and is very sweet.  Her family really keeps to themselves and to be honest we haven't gone out of our way to make friends.  They also do not speak very good English.  Sooo... this little girl, Z, acts very nice to my face.  She is not so nice though.  She has threatened my daughter with not being her friend too many times to count.  She has made my daughter cry too many times to count.  She has lied about tons of ridiculous things like coming over to knock (when she knows K is not home) at like 10 at night.  I've been in the living room the whole night and know she never did... not to mention our dogs never noticed either!  She can also be very nice to K, which makes the situation even more hard and confusing.  Her parents are hardly ever home and in some ways I feel bad for her... but she is very "well" taken care of.  I, truly, do not think that she is a mean girl or that her parents are awful... nothing like that. 

Tonight, I worked at church like I usually do for a couple hours on Wednesdays.  Mike told me that K, Z and her brother were selling stuff and had made lots of money!  My daughter is definitely a little entreprenuer.  Anyways, they were getting everything to sell from our house... granola bars, bird seed, snacks, etc.  That is fine!  But... when it came time to come inside no one helped clean up (which is another ginormous issue we have with them) and Z hurried away with all of the money.  She also mentioned that she couldn't play the rest of the week "because she had plans"... well this girl "has plans" constantly but will knock the next day!  K came in crying because no one was helping her clean up and because all the money that she had worked so hard for (yes, she was the one doing all the work... holding up the sign, waving people down, taking orders, everything) was gone with Z. 

We tried to tell her exactly what to say to Z in a loving way as if Z had not meant to leave with the money.  Well, K would not!  She started crying harder!  So my wonderful (but sometimes a little short tempered with Z) went and told her that I was going to split the money.  Of course, I had to split it 3 ways even though it was all our stuff.  Fine.  No big deal.  There were several other issues with the money, like our daughter putting some in the "pot" before it was split, etc. 

Where I need HELP!!!!! 

How do I teach my daughter to stand up for herself?  She is so afraid of not having anyone to play with that she puts up with junk like this.  Z can be nice and they play great most of the time, but when she doesn't she doesn't.  We explained how Z was not being honest and was trying to decieve K.  We talked about how we sometimes become like our friends and how those are not good qualities.  Would you continue to let them play?  How can I teach my daughter that it is better to sometimes have no friends as long as you are being true to what's right and what is fair?  I feel like she is learning that it is ok to put up with people who treat you badly (when you have a choice). 

Thanks so much for the advice!!