[Rant] Skin Reaction, Problematic Client Experience

2 years in tattooing this is my first experience dealing with problematic skin and client/person.
Sorry for my bad english but i'll try to sum it up:

TLDR:
- Client wanted an A4 sized tattoo on his back
- Told him the S.O.P like no alcohol 42 hours before,get enough sleep, etc.
- It is really hard for the ink to stay in the skin. i had to change needle few times and experiment a bit
- Some rashes appears while in the process of tattooing
- 'Unfinished' result (sorry i didn't take any pics,my mistake)
- Asked him to check to the doctor/dermatologist immediately but i smell a lie from him. he told me 'that's just scar'. Asked him where he got checked up but he didn't answer.
- 6 months later , he asked for a touch up. I asked him for a touch up fee and check on his skin first because i need to know specific what's his problem. rather self diagnose it because i don't wanna ruin the skin for the second time.
- Client 'refuse' to check on his skin 'problem' to the doctor/dermatologist and pay the touch up fee because he don't have money.
- He said that if he knows that something like this will happen, he didn't wanna get the tattoo in the first place? i'm confused because we tattoo artist can't even predict skin reaction right?
- Client told me that i'm unprofessional to one of my friend
- i got blocked LOL

Just wanted to share this and i need advice if this something happen again.
I know maybe i have to build a consent form in the future regarding health/medical condition.

Thank you!

Update: i found out that he have somekind of HPV/warts under his eye. i don't know if there's any health/skin correlation with his back.
idk i'm just guessing there's already 'something' beneath the surface

Stuff like this can be tough but I tend to do the touch up for free as it’s my reputation walking around on the client’s body. I factor in one free touch up for any work other than hands, fingers, feet, and probably end up doing 1 in 15, though often it’s when they’re in for their next piece.

I know we all have to make money but I try to put money second after the quality of the finished tattoo, I think the other way can do more damage than good overall.

Pity you couldn’t present the issue in photos, would have provided better context and perhaps generated more specific advice.

If you’re doing good work and your clients are happy most of the time, move on and put this one down to experience, everything should be considered on balance.