Negative friend--how to make her happy?

I think you should tell her all of what you just wrote here. Also I think you should challenge your friend to be single for like a month (offer her something like a day where you do whatever she wants you to do, or like 100 dollars, or a meal at her favorite restaurant where you pay) and make her write about how it feels then decide if it was better being single or in a relationship. Also I think you should help your friend figure out what qualities her perfect guy would have (and you guys could make a list) then make her promise to only date a guy who has all the qualities on the list. Also get her to everyday say one good thing about that day and try to convince her to write a gratitude journal where she list everything she is grateful for.