Is it OK to say "The guy that helped me, who is 25, is friendly."? The adjective clause in commas can be omitted, so can I include THAT in the subject?
You may say such things, but it is confusing for a native speaker to parse.
Is the guy 25? Is me 25? Who is friendly?
The guy who helped me is a friendly 25 year old.
I was helped by a friendly 25 year old guy.
This sentence is grammatically correct but I wonder why you are saying it? What is making you add that he is 25? What does it have to do with him being friendly? The problem with your sentence is not grammar but content and I would split it in two or rephrase it as already syggested.
Here is an example of a sentence with the same structure which is a lot more logical:
The guy that helped me with my homework, who is a much better student than I am, got an A and I got a C. The teacher must have realized I did not do it on my own.
Here it makes sense to add "who is a much better student than I am" because it explains how the teacher knew he deserved a better grade.
One more example:
The teacher that arrived yesterday, who is supposed to teach a boring subject like Math, was wearing some really funny shoes and joking during the entire lesson. I think Math will be fun this term.