Just as an example...my last horse Nico
Bought for $300...Papered QH with running bloodlines. Great looking, good conformation...green reclaim as in he had been trained but sat for over a year without so much as a brush turned on him.
It took me 7 months to get him really going well. In the end, he was able to run a great pattern, trail ride, do basic lateral moves. Started him on jumping as something else to do with him. Could have done well in most western disciplines but wasnt built like your average "english" horse though his movement was spot on for dressage.
I sold him last week for $1400. That was more than what I was asking because I had two people who both wanted him. That is RARE.
So, on the surface you would say I 'made' $1100 on him, right? WRONG! His board for 7 months was more than the $1400 I got for him. Not to mention trimmings every 6 weeks (4 trims at $40) $160. Vet bills for shots/coggins/teeth/bloodtests $400. Time...I can't even get into how much time I spent working with him. Specialized feed $300 over 7 months.
In the end, a horse that on the outside looks like I turned a profit on...I actually lost quite a bit of money on. I don't see it that way as he was never intended to be a 'flip' project. I gained a ton of experience working with him and learned the ins and outs of a hot horse. And I loved him...thats priceless. I sold him because I lost my "real" job and needed the cash. Its a cruel world.
I wish your sister and her friend best of luck but they also need to consider that not many people are going to pay top dollar for a horse that a 12 and 15 year old has trained. Its mean to say but its the truth.