First of all, you don't tell her that she won't see her preschool friends. Don't even bring it up! If she wants to get together for play dates, then set those up. But don't impose on her any emotional reactions that she may not even have. Saying this is "breaking her heart" is projecting emotions on her that you, truly, have no idea she will experience!
Get involved in the fun of summer, whatever that means to you. Sleeping late? Going to the beach? Going to some sort of summer camp or the library programs or town recreation activities? Going camping or on vacation? Doing driveway chalk and hopscotch and putting in a wading pool? Gat a membership or borrow passes from the library for a local children's or science museum?
You'll be surprised at how fast kids' attention spans switch around! Life is about new experiences, not lamenting what one has outgrown. Be more positive.
If your concerns about preparation are based on few social skills, consider holding her back a year. But if you think she's on target and will just be meeting new kids, get excited about the kindergarten orientation and all the fun she will have. New kids, new classroom, new resources, new fun...positive, positive, positive. Go to the school during the summer to play on the playground or run around the bases, whatever makes it feel like "home" and a fun place. Point out the school bus going by and say how cool it will be to ride that. If she's never been on a bus, if possible, go downtown or to a nearby city with mass transit and ride a bus or a subway/trolley. Make it clear that her bus will only have kids and not a lot of stops, but gee whiz, isn't it fun to look out the big windows?
Stop trying to set her up with lifelong friendships at age 5. How many people from your kindergarten class are you still hanging out with? How many kids from 2nd grade do you remember, follow on Facebook, send Christmas cards to? Instead, boost her confidence in meeting new people and growing and finding relationships as she matures. You want to raise a child who is totally competent in going into new situations, meeting a new teacher each year, going off to sleep-away camp or college, and more!