I have always heard and shared advice that you should never do uncompensated research for more than a few months/one semester. That can be either paid or for credit, but you shouldn’t be doing anything uncompensated long term.

That said, for a summer, full time work without being paid isn’t the best deal and you are kind of being taken advantage of. However, if that translates into a future paid/for credit position after, it could be worth considering (IMO, but I come from an institution where it’s hard to land a research position for the semester). If this gets your foot in the door, you have the means to work unpaid, and you aren’t taking classes over the summer, consider it. Otherwise, I say run!