Just ask to be paid by the day/hour. Look at salaries in your city for 2 years experience, and ask for the upper end.
Negotiation is simple: ask them to make you an offer first, if they offer you less than want state your price, and if they wont meet it then walk away.
If he takes the job, then this is certainly wise. But it is still most likely a bad idea to take the job.
* As an undergrad on summer internships, and in your first few years of professional work, it is better to optimise for learning rather than earning. A project where you work alone with unfamiliar technology at a non-software company won't provide that. He should instead find a software company with a culture of strong mentoring and code reviews.
* It's possible to learn Android, iOS or web development over a summer if you have some previous C++ development experience. But learning all three, and designing and building a reasonable quality app on two platforms + a server backend is just not feasible. The end result will be poor, and the client will not realise that this was because of their own unrealistic expectations. As a result, OP may not get a great reference to use after graduating.
I mean, if the other alternatives are fast food jobs, then sure. But he needs to manage the expectations of the employer very carefully - maybe proposing a mobile website only, no native or hybrid apps for the first version.
Just ask to be paid by the day/hour. Look at salaries in your city for 2 years experience, and ask for the upper end.
Negotiation is simple: ask them to make you an offer first, if they offer you less than want state your price, and if they wont meet it then walk away.