I get what this service is trying to provide. However, I think family members, and the let's play community have provided an alternative for the impossible to compete price of free. Honestly, I do not personally see the appeal of downloading mainstream games, or purchasing stuff for the consoles we have that I'll have to struggle to complete on the lowest difficulty. I don't have the time to fiddle with inaccessible games these days, nor the disposable income (Well technically I do, but paying $60 for a fairly inferior experience compared to what a sighted person would get from a game does not quite justify the purchase). This also means that I don't have income to give to other people to play part of a game for me. And as some people posted earlier, people would most likely refer to an immediate family member or friend to help them with stuff like this as one gets a personable and better experience than having a stranger do something for them.
Also, to answer the question of why people are probably mobbing on the OP on the idea is that they should not waste their time with this approach with all the trimmings being offered. Considering the topic trend so far, the market for this is most likely limited. You'd probably get better business for creating mods to make a game playable for the visually impaired than having people pay you to play a game for them (though to be fair, this is illegal as you're making money off someone else's intellectual property but the idea is one would more likely be profitable than the other).
Also for the examples of what people wanted to use this service for, everyone does see the posted rates. We're aware on how long some games are getting with their content, and I believe that a person who would purchase a product would want to do everything possible with the game. I've seen people wanting an entire game which is nearly impossible to play be put through this service. Your enjoyment of the game will be limited on how much money you can afford to throw at this person to play the game for you. Once you start running low, you either have to cut your enjoyment short by having the game end, or end on a cliffhanger as you cannot afford to pay any longer. Now realize this is for one game, and you most likely need a bit of help with a bunch more. Considering $60-$40 price tags on most popular titles... Good luck affording that (refers to the unemployment rate of disabled workers in a few countries).
Though for the naysayer crowds; Why aren't you being constructive with your takedown of this idea? Obviously there are some problems to make their current model look nonviable as a small business idea, but you can at least throw in something to actually improve the idea. Honestly, i don't see the trophy thing actually panning out in the long run as that will take some time (queue requests for someone to platinum a game for you). There is also a workload component to think about as this looks like a 1 person job. It might be better to just focus on the quickest possible thing the OP's friend can do for people, and that is quick helps with unplayable sections, most likely as a subscription model for the game. For whatever rate per month, people can send you a save file or whatever that requires you to pass a small section, or do a bit of allocation work. Then send it back or whatever. You agree to do these small things to get past some annoying interfaces, but each request must be doable in 30 minutes. This helps limit the workload on your friend, and assures he has time to work on each game without being overloaded with massive requests.
...Also you know, for nightmare level requests that might be hard to actually do (can you play all of Fallout 4 for me on survival difficulty without any mods?) (Note, Where saving is limited to beds, dirty beds can lead to illnesses earned, and dying resulting in hours of work erased and certain spots being far from beds... also being killed in like one-two hits... and the need for food, water and sleep... and no fast travel... and your inventory carry limit is reduced harshly... and oh god please don't ever agree to a request like this).
...However, if the OP and his friend already know this probably will not be a massive thing (and hope that it isn't for workload's sake), and requests will be small... then what the hell, let people throw their money into this service. As the old adage goes, "It's not my money being wasted."