Has he used starter heat wrap like this? They help with heat soak. When you have issues hot, but not cold, this could certainly help.
https://www.summitracing.com/parts/sum-350118-1
Has he used a gear reduction starter?
Is he using QUALITY batteries? I had 'starter' problems for years that turned out to be a cheap battery. Bought a 750 CCA battery and it cleared up some of my problems. Might watch voltage drop at the battery while cranking.
Is every connection clean and tight?
What grounds do you have? I added a second ground wire from the battery to the case of the starter. There is a stud on the back of the starter. This seemed to really help.