How to give EDIT permission for one user in only one topic?

[Lyvyoo]

Hello,
I search the forum but I don't find the correct solution.
I want to give a permission to a user for permanent editing of all posts (including mine), in a specific thread in forum. So, only one topic, not an entire forum or child forum. The user is not moderator and I don't want to be.
I installed Custom User Permissions plugin but it lets me set editing permision for a specific user, only on entire forum or a specific child forum, not in a specific thread.

Thanks for help,

[s3_gunzel]

This’d require a plug-in.

[Lyvyoo]

What plugin exactly?
One way that I found is to change the user group from "Registred" to "Moderators" and edit permisions in order to let alowed only the edit function. For me is not a problem because I don't have moderators on forum but in this case the name of the user will apear on top of the child forum like a Moderator. And I don't want this.

[Ashley1]

A plugin would have to be created for you. May not be difficult, but i don't think you can do that with default permissions. You don't have to do that, what you did with the moderators group - rather think about what you need from that group and then go apply it to the registered users group.