I paid a little more than that to my lawyer to get my permanent DNI, starting almost three years ago (I know the price has gone up quite a bit since then, and many lawyers back then charged as much as three times what I paid). Half on completion of temporary residency, other half on delivery of DNI. It just so happened that my process took so long (due to my issues, not my lawyer's) that I received a permanent resident visa as my first DNI. Heh.
In my opinion, it's not necessarily a bad thing you're friend is looking at.
Steve is right though - they can't do everything for your friend. He'll have to go get his fingerprints. He has to be there the day he applies. Other times here and there if there are issues to deal with. But everything else my lawyer handled for me - he did ALL the legwork for me, including getting my FBI report (I got the fingerprints but he did the rest, including paying for it and then charged me as an extra fee, all agreed up front) and reviewing all of the documents, telling me where we needed to make adjustments, taking them to his contacts in immigrations and ensuring there were no snafus, etc. He was always dependent on following the process up and making sure that things were moving along. He even ended up picking up my DNI from the distribution center, even though you're supposed to have to do that in person.
Steve and I have a difference of opinion on whether or not to use a lawyer but he had a much easier time of it than I did. It was worth the money to me to be able to work and concentrate on my daily life than worry about all that.
If they tell your friend they can do all those other things that we all know he's going to have to do personally, watch out and try to see where the scam is.
The rentista visa - I don't know that it's true they don't do checks to verify things. I have a friend who is getting a rentista visa through a company he owns in Panama. He had to have accounting reports (which he got himself through his accountant in Panama and passed on to the lawyer) showing that the company is a going concern and has money to pay the monthly requirement. In order to qualify the money for the rentista visa, it basically had to come from an investment - he was a part owner of the company and the money that would qualify were dividends paid by the company to him as a shareholder. Immigrations here DID follow up on his stuff, including rejecting his first application and making him make a second that hasn't been ruled on yet.
There are various "gray" means to meet the qualifications of a rentista visa and I don't know - maybe this company your asking about provides something like that?
If there is a problem with whatever process they use, I doubt you would go to jail, but you may not be able to apply for any other residency as a result and may even be shown the door - I don't know.
As far as using some sort of facilitator who has contacts in immigrations and can get the job done with as little headache for you as can be managed (there will be some, it is completely unavoidable to at least some extent) for $400 US (I'd pay him dollars at a 5-1 exchange rate if you can work it out with him), I'd try it, particularly if you don't have to pay anything up front.
BTW - do be prepared to pay all the fees that you have to pay for application of residency, application for DNI, various fees here and there, etc, that have all been discussed before. I doubt very much the price includes these items.