Recurring Exertional Rhabdomyolosis (ER)

Hi all.
I’m currently training for a MTB marathon stage race with a friend who is anything but structured with his training. He is a tough individual and has the ability to keep pushing long after my head tells me to dial it down, or else self destruct.
The problem is that he’s been struck down three times with pretty serious ER in his legs that manifests a few weeks into intense bursts of training. He’s been to various doctors and they haven’t pinpointed a cause.
I’m not a doctor, but it seems pretty obvious to me as someone who tries to keep up. 2-3hr rides that turn into 4-5hr rides up mountains with 2hrs of water and snacks. That’s the last ride we went on.

Has anyone here experienced something like this and could they offer any training advice? It seems this is not that uncommon with endurance athletes and the ability to switch off the discomfort can go too far sometimes.
I’m not after medical advice, as that’s best left up to the professionals. Although, I feel I might be able to help him address an issue that might be related to his on/off training structure.

I’ll be gutted if he doesn’t make it to the race with me.

PS. Then again, it would be awesome to have a support person.