One thing that might come up that's worth thinking about is IP. Giving that it is your senior project, you've had 4 other people working on it, and a co-founder who is leaving - who owns what share and are they going to give it to you for you to work on are good questions to ask.

Further, you say that it's too much work for you and your co-founder is leaving. Which means by turning down the job you are committing yourself to too much work that you acknowledge is unsustainable for you.