Hey guys, need your advice please.

In september Last year I've started my apprenticeship. Before that I made a year internship and developed a Software for them.

After I've left the internship, my Old Boss (Boss from internship) asked me if I can support the Software a while. So, I set Up a git, made it available in github, but private (had to paid for it - from my own Money). After not hearing something from him - He didn't understood how He Can Log in Into GitHub -.- WTF ! I wrote him multiple instructions but it was useless.
Because I don't have the desire and patience anymore - and don't want to pay with my own Money to Make it available for them - pay for something they can't even Login because they are ... Yeah.. I've decided to cancel the membership in github some weeks ago.

Today my Old Boss contacted me via E-Mail, after not hearing something from him in months.

Now I don't know how to react - He wants that I Code for him, the Software again.
on the one Hand I don't want to "leave him alone" but on the other hand I don't want to Support this Shit anymore.

What should I do?
At least I want to get paid for my Work. But I don't know, if this is legal to earn Money next to your apprenticeship. My current Boss Said one time that this would be No Problem, but I'm not sure about it.

Would be glad about any help and advice from you.

Thanks.