I just Googled for
average cost of developing an app and this page said:
Apps built by the largest app companies, the "big boys", likely cost
anywhere between $500,000 to $1,000,000. Apps built by agencies like
Savvy Apps cost anywhere between $150,000 to $450,000. Apps built by
smaller shops, possibly with only 2-3 people, likely cost anywhere
between $50,000 to $100,000.
That was dated 4 Feb 2015, so prices have probably risen.
I have known a bunch of people who were sure that they had a sure-fire idea that would make them a million.
Not a one would have been willing to invest the lowest sum mentioned, $50k, even if they had it.
Because I enjoy developing software, and would be doing it as a hobby anyway, I have twice agreed to do all of the development, while “idea guy” did all of the marketing. Note that I only dealt with people in an industry with a client base in that industry, who could reasonably be expected to bring in sales.
Since neither of us knew at that point how or if it would work out we agreed on a 50-50 split. If you will need a marketing guy later, do not short change them – give them an equal share. Probably best to find one now, to see if the idea is even feasible.
I won’t get into the inevitable delays, overruns, ideas for “minor, but must-have” features that took months to implement. Suffice it to say that no friend of mine could have afforded to pay my charge out rate for the time I put in, so a percentage seemed the only possibility. YMMV.
Be aware that some people have extremely vague postcard pitch ideas (“like Facebook, but better”) and think that that is all that is required of them and you will make them a millionaire in return for a few $100. Don't bother trying to persuade them; just make your excuses and wish them good luck.
See also https://freelancing.stackexchange.com/
[Update] To actually address the question: I have had more opportunities than those, but decided not to pursue them. The most important thing for me was the personal aspect - whether I thought that I could do business with the stranger with the idea.
What your friend should beware of is that during the development phase, he has the upper hand and can call most of the shots. for me, that translates to me taking the technical decisions, but explaining them to my partner. Similarly, when he makes the inevitable requests for new features, he has to explain the business case to me.
After the software is ready, the balance of power shifts, then the final marketing decisions are all in the other guy's hands. I have taken care to choose partners who will consult me and listen to my input, but ultimately, I have to trust that they are the expert. Which is why it is so important to choose the right partner.
I am not sure if that is cynical, or rational, but it's my view and YMMV.
To finally answer, once we knew that we would have such a relationship, there was no problem being straightforward. I explained that I would be investing what would be the equivalent of 6 figures of my time, evenings and weekends for months, in one case years. And, as sure as people are that their ideas are sure-fire winners, there was no guarantee of success. It might make us rich, or it might tank, so how about an even split? In both cases where I went forward, my partners agreed. In at least one case where the other person held the view that "it's only a little bit of software", I wished him the best and told him that I hoped that he would would find someone who would do the work at the split that he envisaged and we parted company.