All of the above answers are wrong.
First off, forget about 6ya! Sixya, fixya, scam artists all. See for example, https://tinyurl.com/ybpds4h5.
Calling them for a digital piano repair would be like calling a plumber to do brain surgery.
She's asking about a KORG C-155. Not a Roland anything. Not a Technics anything. Not a Kurzweil anything. And the problem isn't about dead notes or intermittent loss of touch sensitivity, or volume issues. Her piano is dead until after a 15 minute warm-up and then plays NORMALLY! This issue just about screams, "POWER SUPPLY PROBLEM!" At least, this is the first place to begin.
For the average user, this is not a DIY project. Troubleshooting it will require an experienced technician with the proper instruments and tools at least an hour to open the unit (ker-ching!), take some measurements, identify the defective assembly or component. Then on a 25 year old unit, he would need to start hunting for replacement part(s), since KORG very likely no longer stocks parts on a unit that old.
That is probably why the typical answer from a professional would be, "save your money, and buy a new unit." Because of the parts issue, the cost of the repair will likely be in the hundreds of dollars (ker-ching!), and when it breaks down again with something else (ker-ching! ker-ching!).
I'm not saying the piano can't be fixed, but I am asserting that it can't be fixed economically by a professional who cannot afford to spend many hours on isolating a defect, hunting online for a USED replacement assembly, or a NEW component (assuming that the repair involves just one or two individual components), then actually doing the repair, and putting the unit back together, and only get paid for a fraction of the actual time involved.